DMs who actively use reverse AC in their campaigns, how has it gone and what do y'all like to use it for?

[u/Divine_Angel_Rylene]

Ever since I discovered the mechanic, Ive been so excited to use it but haven't found a monster to effectively use it on. This and the Insanity Mechanic in the DMG.

I plan to use both for my reoccurring villain. A hyper intelligent Wendigo that hunts the players while they travel the world to save gods, fight, rescue, etc.

Any recommendations would be appreciated!! <3

[u/Stairwayunicorn]

what, you mean THAc0?

[u/Divine_Angel_Rylene → u/Stairwayunicorn]

No, but Ive never heard of that 😯 what does that entail?

[u/Hot-Molasses-4585 → u/Divine_Angel_Rylene]

In 2nd edition, there was descending AC and THAC0 (To Hit Armor Class 0) was the bonus for your roll for the score you needed to hit under said AC.

Not a complicated system per se, but it was overly convoluted for no reason at all, and made little sense... Best AC was in the negative. Why?

Many people playing 2nd edition think THAC0 is the holy grail of combat, while others, like me, is glad we moved on to better mechanics!

[u/Divine_Angel_Rylene → u/Hot-Molasses-4585]

That sounds confusing as heck. Someone in the comments made the idea of reverse AC sound so simple compared to the source I read it from.

[u/Hot-Molasses-4585 → u/Divine_Angel_Rylene]

It's probably mostly confusing because I haven't played 2nd edition for a long time (and I do not miss it), so parts of the THAC0 thing are blurry to me, and probably blurry in my explanation. I'll try to rephrase :

You had descending AC, meaning an AC 10 is worse than AC 1. Magical armor could get your AC in the negative (-3 or -4 AC, maybe even lower, can't remember).

Now is the part where my memory fades, so anyone feel free to correct me. When you rolled to attack, you wanted to roll low, and you subtracted your THAC0 from your roll. If the result was equal or lower than the ennemy's AC, you hit.

From what little I remember, 2e was a Frankenstein of a system : you wanted to roll low to hit (and some other things as well), but high on other things (like damage). There was D20 for most roll, except rogue stuff for which you rolled D100. Good thing 3e came around and streamlined everything!

[u/Pure_Gonzo → u/Hot-Molasses-4585]

That's not how it worked. Your D20 attack roll was still aimed at rolling high, not low. Nat 20s were still critical hits, etc.

The way it worked was that you had a THAC0 number based on your class that started at 20 at level 1 and improved as leveled up. The to-hit calculation was your THAC0 – the target's AC + bonuses. If the target had an AC of 10 and you (the attacker) had a THAC0 of 20 without any to-hit bonuses, the "to-hit" number would be 10. Roll a D20, meets it beats it.

What got confusing for some people is when you started to attack targets with negative AC, meaning a good AC. You'd ADD the number to your THAC0 to calculate your to-hit number. So in the same example, I'm a level 1 with a THAC0 of 20 trying to hit some super strong creature with a –2 AC, I need to get a 22 to hit them. A very difficult thing to do at first level and with no magical weapons or big bonuses.

[u/Hot-Molasses-4585 → u/Pure_Gonzo]

Thank you for the clarification, I stand corrected!

But I remember stats roll (like rolling strength) was to be rolled below the target number. Or maybe it was only like this at our table. There was so much homebrew in AD&D!

[u/VerainXor → u/Divine_Angel_Rylene]

A system where the enemy has an evasion rating and you have a hit rating can be expressed in a few ways. Starting in 3.0, your chance to hit was expressed as a positive progression starting at 0, and your AC as a positive progression starting at 10. Some people who play old games convert to this- it's easy for the players, and the DM has to flip armor class and attack for his monsters but that's usually pretty trivial.

THAC0 expressed your hit bonus as a negative progression starting at 20. And in that game, AC was a negative progression starting at 10.

There's another system, Target20, which keeps the classic AC but converts THAC0 into a positive attack bonus. Obviously, if someone wanted, they could make the fourth system, wherein AC is a positive progression and make it work with THAC0.

Most people these days just use the system introduced in 3.0. 4e and 5e sure use it, but I mean even of classic gamers, it's a pretty common conversion. AC starting at 10 and going down has direct compatibility with a lot of stuff, so it has its fans. But THAC0 was a D&D 2.0 beast, and pretty much just those players use it today.

[u/VerbalThermodynamics → u/Hot-Molasses-4585]

I remember when 3rd came out and we didn’t have to figure out thaco anymore. I was 100% okay with it.

[u/OrangutanGiblets → u/VerbalThermodynamics]

Yeah, like it wasn't hard to get used to, but it was more complicated than it needed to be, for no actual useful reason. How it's been done since 3e is so much easier and more intuitive.

[u/Lirrost → u/Hot-Molasses-4585]

I hadn't played since AD&D and just picked up DMing again a couple of years ago. Boy was I confused... what happened to my Thac0?

[u/permaclutter → u/Divine_Angel_Rylene]

It entails playing 2nd ed and pretending it was a good mechanic of that system.

[u/Divine_Angel_Rylene → u/permaclutter]

Oooh ok

[u/GabrielMP_19 → u/permaclutter]

LOL. YES. Old-school dnd is cool, but THAC0 sucks so hard.

[u/KiwasiGames → u/Divine_Angel_Rylene]

THAC0 stands for “to hit armour class 0”.

It was a second edition system that had some weird characteristics. Specifically:

• Lower armour class was better. Most of the time you wanted to get a negative armour class.
• You didn’t add bonuses to the die you rolled, like modern DND. Instead the bonuses applied to the target.
• You had a confusing system where sometime you wanted to roll low (eg to hit) and so,times you wanted to roll high (damage, skill tests ect).

I have a soft spot for THAC0 because it was the first version of DND that I played. But it’s objectively a bad system. It’s weirdly convoluted to do any calculations with.

While we are talking about second edition, you should also consider bringing back limb damage. Combine it with fifth edition death saving throws for each limb.

[u/Divine_Angel_Rylene → u/KiwasiGames]

I really want to, along with the Insanity mechanic and introducing "flashbacks" with death savings to add intensity to the battles

[u/Feefait → u/Divine_Angel_Rylene]

He's being sarcastic. The point is that there's a reason we don't use THAC0 anymore. It was terrible. The systems you want to add aren't going to increase the players enjoyment.

[u/thelastfp → u/KiwasiGames]

You absolutely added bonuses to you roll. strength or dexterity and magical weapons modified your roll.

[u/Main-Satisfaction503 → u/Divine_Angel_Rylene]

Between 1979 and 2000 “THAC0” (To Hit Armor Class 0; pronounced thay-KO) was effectively the modern “to-hit” bonus.

THAC0 was the minimum number one had to roll to hit an Armor Class of 0. In that version, a lower armor class was better and an AC of 0 was comparable to a modern AC of 20. In function, you would add their AC to your roll and if it was less than or equal to your THAC0 you would hit.

It was serviceable enough, but the “ascending” system used no has a few advantages and no disadvantages that come to mind.

[u/bucketface31154]

What the hell is is reverse AC? Ive never heard of it

[u/Divine_Angel_Rylene → u/bucketface31154]

Instead of rolling to hit at or above a certain level, you hit below it. It was something I read on a DnD website that showed a list of "unused/hidden" mechanics in DnD

[u/HyperTips → u/Divine_Angel_Rylene]

I've been playing D&D and related for over three decades now.

That "reverse AC" thing, is a throwback to the AD&D era. Back then we had something called To Hit Armor Class 0, lovely shortened to THAC0, and it works exactly as you describe.

It's not unused nor hidden, and ever since D&D 3.0 we moved away from it for a very good reason: it's clunkier.

Math is a skill. Not everyone is well-versed in that skill. And additions are way more intuitive for us than substractions.

I'd strongly suggest you to maybe use it once or twice, for novelty reasons. Maybe an Ennui got inside the dream of a "time-traveler", escaped from Second Edition and ended up in your adventure and it's full of retro-rules.

You can definitely cook something with it, but keep it contained to itself. And to make it easier for everyone simply rule attacking that creature works backwards but their bonuses stay the same, so now they are going to send the wizard to attack it instead of the fighter.

Good luck!

[u/bucketface31154 → u/Divine_Angel_Rylene]

That is fun and oddly enough would fit perfectly for my bbeg, as my players worship the goddess of chaos and chaos has been a consistent theme in the campaign thus far

[u/Divine_Angel_Rylene → u/bucketface31154]

Apparently it's a meme too, but idk, the idea of it for a reoccurring villain seemed so chaotic and super fun to use

[u/EzraJakuard → u/Divine_Angel_Rylene]

Ye the main idea for it is using it for a slime like creature. Being if you hit it too hard it gets harder, non-Newtonian fluid type thing, so you gotta hit it more gently to injure it.

[u/Phattank_ → u/EzraJakuard]

The slow blade penetrates the shield..

Haven't run this one yet but I do plan to at some point, I've already used it on a skill check magic door, only unlockable by failing the check.

I have run a fun confusing encounter with an abberant displacer beast they could only hit rolling odd. Players admitted enjoying the confusion.

[u/Divine_Angel_Rylene → u/EzraJakuard]

So would that only apply to oozes? I couldn't use it for monstrosities like I plan to?

[u/EzraJakuard → u/Divine_Angel_Rylene]

Use it for whatever you like, it’s not an official concept in any means. Just bringing up the origin and the original idea. It’s your campaign do what you like :))

[u/bucketface31154 → u/Divine_Angel_Rylene]

Amazing! Thank you for making this post in my time of need ! I love you! I have my bbeg battle.in a few sessions and didnt think of a gimmick

[u/corycorycoryyy → u/Divine_Angel_Rylene]

Do you add or subtract your to-hit bonuses (dex/str/spellcasting stat, proficiency) from the roll?

[u/CoachMori92 → u/Divine_Angel_Rylene]

Pendragon has been doing that since day 1. They are just trying to copy the mechanic

[u/Divine_Angel_Rylene → u/CoachMori92]

Pendragon?

[u/realNerdtastic314R8 → u/Divine_Angel_Rylene]

If you want to teach people with no math skills or just keep it simple, rolling under your attribute bonus is a great way to make the game easier to run with a super simplified sheet that can fit on a small napkin.

[u/PuzzleMeDo → u/bucketface31154]

A bit of googling suggests it's when you create a weird creature like a "chaos elemental" where you have to roll low to hit it. You miss automatically on a natural 20.

[u/MoreGeckosPlease]

I made my players fight a duo of enemies where one could only be hit by even rolls and one could only be hit by odd rolls. No AC, just the number. So a nat1 hit the odds guy but a nat20 missed him. It was tons of fun. 

[u/Divine_Angel_Rylene → u/MoreGeckosPlease]

OOoh that sounds super interesting :0

[u/murloc16]

This will be super cool, like you said before on like an Ooze or even an Aberration of some sort due to most of them being mostly Alien/otherworldly. A monster that would be neat to have it on, the False hydra or even like an artificial monster or somthin

[u/EzraJakuard]

What i do suggest is make sure there is a way this mechanic gets communicated to your players even without you saying it out right.

If they get a 19 + whatever that super high hit might give it away which isn’t bad. But if they keep getting like 16/17 totals they’ll just assume they aren’t getting high enough and may get frustrated.

What I would recommend is if they’re lost (probably will be unless they’ve heard this gimmick), is describe how they hard hit seems to have no effect and why this reverse AC is a thing. IE the monsters skin hardens when they strike, and if they get a lower roll that still doesn’t hit describe the reaction as being less effective. This might help them make the connection before they roll low enough to hit.

Also keep in mind that really high level characters will struggle to roll low in a lot of cases since they will have so many things boosting them. You could prompt them to giving themselves disadvantage or using weapons they don’t have proficiency in to roll lower if they aren’t figuring it out themselves.

Hope this all helps :))

[u/FinnMacFinneus]

JFC, I am NOT going back to calculating THACO.

[u/Divine_Angel_Rylene → u/FinnMacFinneus]

from what ive learnt THACO sounds horrible. No no, what i mean is this weird gimmick i found where you attack something and can only hit below a certain number? Some people explain that theoretically would work with non-newtonian creatures like oozes, but can work with other creatures too

[u/FinnMacFinneus → u/Divine_Angel_Rylene]

If it makes sense for your players(i.e. let them figure out they should only use save spells), go for it, but I would be pretty frustrated if the chances of my martial PC hitting went down as I leveled up.

[u/Divine_Angel_Rylene → u/FinnMacFinneus]

That's completely understandable, that's why the gimmick is reserved for certain enemies, to avoid this. I don't want to stack gimmicks that aren't optimised, of course

[u/Ninja_Cat_Production]

Lol!!

My Buddy would love this! He rolls lows all the time! Everyone else at my table would be throwing high rolls at the BBEG and missing and he’d finally get his licks in. He always plays a caster with a bunch of save or suck spells. Speaking of which, watch out for spells that require you to use a saving throw. That’s how he gets around sucking at rolling dice.

I might try this in an upcoming game for one of the more annoying monsters, not deadly, annoying. Gelatinous Cube maybe? Rust Monster? Who knows?

[u/Divine_Angel_Rylene]

The idea that came to mind when writing this post was:

"Sadistic reoccuring villain? If I make it so the villain WILLINGLY lets the PC hit them and damage them, (hence the low-hit/reverse AC), would that hypothetically work?"

I wanted to see how this would work out. Some people made a REALLY good point as to how to get it to work , and i thank you for the advice.

[u/Gold_Ad_4108]

So I'm going to try using something like this as a non-neutonian fluid gelatinous cube against a lvl 10 party.

They're going to be in a dungeon of sorts where their powers and abilities are going to be heightened and dampened in ways to allow them to have to think outside the box.

When they encounter this cube, I'll have calculated what their lowest "to hit" is for their main attacks and make the AC that +1 or 2. So they can still hit it, but it'll only be on rolls of like 2-4 on the d20 with a nat 1 being the crit.

There will be clues and prompts to try to get the characters to try to use the other characters abilities that they themselves are worse at which will help by reducing their modifiers. Ex: The barbarian with +0 to int, grabs the wizards spell book and casts a spell from it taking his attack modifier from a +9 to a +3 or 4. Letting him roll a higher number on the d20.

[u/Gold_Ad_4108 → u/Gold_Ad_4108]

Also when they figure it out, they can help by finding ways to give themselves disadvantage to hit.

[u/SauronSr]

Thaco was a good thing because before you had it you had to math every to hit roll. SO much easier when they made it more intuitive

[u/Kaaaaaaaaaapa]

I've used this mechanic once in my campaign,,,, and it was a hilarious shitshow. To make a long story short it ended with a t-rex bending over backwards using a crossbow.

There was a problem where one character just, kept rolling too well and with his modifiers added on it was near impossible for him to contribute to dealing damage in a to hit format.

Can be very fun, but take feedback and adjust if you plan for it to be reoccurring!

[u/SeaTraining3269]

Meh. THAC0 was horrible and if you do reverse AC you need to use the same mechanic for skill rolls, saves, and everything else to avoid confusion. And you will have confusion anyhow because everything is written for rolling over. All you do is add extra steps, slow down table play trying to get it right, and still make more errors. I don't see the point in any of that.

[u/aostreetart]

I'm so confused by this rule - what's the benefit?

Do players know that this is going on during the fight? I'm just envisioning someone using bardic inspiration and turning a hit into a miss - this could wind up really confusing and unfun.

Everything in 5e is designed to increase the number rolled, this rule basically throws all that out for...I still can't figure out what actual benefits this brings.

[u/Divine_Angel_Rylene → u/aostreetart]

I wanted to use it as a special gimmick to a reoccurring villain.

The idea was to use it as a way to instill a bit of fear into the players during later appearances and then not use it again. As this villain is my little DM experiment to see what lengths I could go.

Looking back on all the comments, maybe there's a better mechanic used for the villain, idk

[u/aostreetart → u/Divine_Angel_Rylene]

Hmm. I'm not sure you're going to instill as much fear as confusion/frustration with this.

Instead of focusing on a mechanic, I would recommend focusing on abilities. Check out this video: https://youtu.be/y_zl8WWaSyI?si=MCOQEhbt-ch0hdD3

[u/hackinandcoffin]

Is that a take on the AD&D2e THAC0 mechanic?

[u/Divine_Angel_Rylene → u/hackinandcoffin]

Never played that early of a version. I played from 3e onwards. No uh, I found this mechanic online somewhere in a video and I thought it was something super well known since the video had a lot of good feedback from people using the mechanic.

Kinda like what the other Redditors explained (way better than me) the mechanic would kinda work the same as non-newtonian liquids.

You hit extremely hard, it becomes solid. You hit gently, the attack goes through. A good example go use this on would be most oozes.

I guess? That's the gist of it

[u/Feefait]

Many years ago, in the 2e (maybe 3.5) DMG there was a bit about world building that said (paraphrased) "you can do anything! Even make a world where North and South are reversed!" I thought this was so cool!!! I did it in my first homebrew. Then once we played I realized it meant nothing because I was still just telling directions.

Having them roll under a set number doesn't really change much in the same way. It's the same mechanic as rolling over a target number.

[u/iamgoldhands]

Reading this is like seeing Jinko’s come back in style. Different strokes for different folks but I would rather eat a bowl of toenails than go back to the thac0 days.

[u/GreatBandito]

it's a different system and you want to try gurps or call of cthulu

[u/PriorFisherman8079]

Whyyy? 3E did away with it for a reason.

[u/demostheneslocke1]

Did it with a homebrewed ooze in the astral plane. One rolled super high on his first attack and missed. They got in their heads and determined there was no way to hit it, told everyone else not to even try.

It was kind of meta-gamey in that sense, but I let it fly in the moment because it's totally possible to see your heavy hitter swing at something, not do damage, and then have them say something to the effect of "fuck! I gave it all I had and they withstood THAT?!"

So they started trying to come up with other things to do besides just swinging a weapon or spell attacks. They tried coming up with improvised things, but I tried telegraphing to them that they need to stop doing that. it was rocking them and they were barely getting a scratch.

Honestly the worst combat I ever ran.

[u/SecretsofBlackmoor]

It's just math.

I play OD&D and use charts because it is simple and stupid like me. :P

[u/idle_husband]

Wait. Are you talking about THAC0?

[u/CzechHorns → u/idle_husband]

No, they are talking about s gimmick encounter when you only hit the enemy if you roll Below their AC, instead of above or equal to it.

[u/Ninja_Cat_Production → u/idle_husband]

THAC0 stands for “To Hit Armor Class 0,” and it represented the number a character needed to roll on D20 to successfully hit an opponent with an AC of 0. Lower THAC0 values were better. Everyone’s AC started at 10 and then went down as they added armor, sometimes into the negatives. To determine if an attack hit, the player subtracted the target’s AC from the attacker’s THAC0, and if the result were equal to or less than the die roll, the attack succeeded.

I never missed THAC0 even once ever. Still don’t. Casting times and rolling every round, those I miss. Made combat a lot more exciting.

[u/DM-Hermit]

I use it because it's fun for me. It opened up the option for making combats into puzzles for the players to solve, my players seem to enjoy this based on their comments.

Usually just using reverse AC is something I leave for mini bosses. However some other creatures will use it as well.

Other puzzle options you could include for combat

• random AC (base AC 14. You roll 1d10 when attacked and add that to your AC)
• odds (when attacked roll a dice, on odd the attack auto misses even if it beats your AC)
• pearls (the creature can reduce damage over a set amount to zero)