Saturday, January 10, 2015

Roll Under Attack Rolls + Players Roll More Dice

I sort of love the whole idea of roll-under mechanics.  So do other people.  I use them in my homebrewed home game for everything from skills to saves to ability score checks.  And now I'm thinking about applying them to attacks.  So none of this d20 + attack bonus, then compare to enemy AC.  I know, sacred cow, barbecue, etc.

I'm also playing around with the idea of making the players roll all the dice.  Or at least, they get to roll the dice when the monsters attack.

The math is totally the same. Characters have the same odds to hit and be hit, but it feels very different, maybe?


A character has a Defense of 9 +/- Armor bonus +/- Dex bonus. Armor bonuses range from 2 (for leather) to 6 (for plate).

Players roll a d20 whenever an enemy attacks. (DMs don't normally roll enemy attack rolls.) A hit results when the result is equal-or-less to the character's Defense value. Defense rolls are made with a penalty equal to the enemy's Attack bonus.
Example: Alice has a Dex bonus of +1 and is wearing leather armor (+2 AC). Her Defense is 12. Whenever she is attacked by a HD 0 creature (e.g. 1 hp goblins), she needs to roll a 12 or lower on a d20 to dodge the attack (because the goblins have +0 to attack). So she dodge's goblin attacks 60% of the time.

If she is attacked by a HD 8 golem (with +8 to attack), she needs to roll a 4 or lower on a d20, so she dodges the attack 20% of the time.

Because you're telling the player the monster's attack bonus during the Defense roll, they'll immediately know what a monster's attack bonus is. This might be fair--it gives players more information about about what they're fighting, so they can make more informed decisions. And it makes sense that a fighter would be able to quickly assess his enemy's skill after a few seconds of combat.

If you don't like telling players what the monster's to-hit bonus is, you can always just ask them to tell you how many points they successfully dodged by, and then compare it to your monster's to-hit bonus.


A character has a Melee Attack of 11 +/- Str bonus + attack bonus.

A character has a Ranged Attack of 11 +/- Dex bonus + attack bonus.
Attacks are rolled on a d20. A hit results when the result is equal-or-less to the character's Attack value. Attacks are made with a penalty equal to the enemy's Armor bonus. 

Example: Bob is a level 3 fighter (+3 to hit) with 10 Str, so his Melee Attack is 14.  He's trying to hit an antipaladin wearing full place (AC +6 = Armor 6), which applies a penalty of -6 to the roll.  So he has to roll an 8 or less to hit, giving him a 40% chance of beaning the antipaladin on any particular turn.  

Pretty much everything that I've said disclosing enemy to-hit bonuses would also apply to disclosing enemy armor.


It's mechanically identical. The probabilities are the same, and technically it doesn't matter who rolls the dice. But does it still feel like DnD?

The question of "should the players roll all the dice?" is something I'm still pondering. On one hand, you lose the sense of symmetry, so that it feels like the monsters don't follow quite the same mechanics as the PCs (though you can argue that's always been the case). But on the other, it's a lot easier to explain and calculate your chances of success (and that's something that Numenera fans are always gushing over).

I'm thinking about implementing this in my home game, just to see how it feels.



  1. I did some similar pondering soon after 5E came out:

    I also looked at how you can tweak roll-under to reflect different roll difficulties, and contemplated having all of the rolls made by players. That said, there are some significant differences in what we came up with!

  2. If the DM rolls for enemies, but rolls the dice in front of the players, they will figure out their attack bonus and AC eventually, after taking at least a few hits, maybe some getting dropped to 0. Having the DM roll is definitely the hard setting compared to what your describing.

  3. One usual reason to favor comparing d20 + bonus to a DC over rolling under is that you compare instead of subtract, which is faster to do.

    Other than that, you might want to read into the system of The Dark Eye. That one is based on rolling under for several stats and using skill ranks to compensate for rolling over.
    Results in some very strange random variable distribution curves and the chance of success is not trivial to compute, so I don't recommend to actually use it, but you might be interested in it as somewhat of an oddity.

    1. Adding is faster than subtracting, but a simple roll-under (without subtraction) is fastest still. I think that's what I want to move towards, where I can say "make an attack vs plate" and the player will look at their sheet, where a single number will be visible.

      Dark Eye, I think, overcomplicates things. I appreciate what they're trying to do (involve 3 stats in every roll, invoke a robust bell curve) but I don't think the extra complexity is worth more than simplicity and speed. It doesn't emulate anything that I want in my games.