In this variant rule, you take defence into your own hands by rolling the dice instead of having a static number. As a DM I personally hate rolling the dice for something that is a player stake and find it more engaging and better for roleplay to know how well you may have defended an attack via a dice number that tells you. It also helps to keep wandering minds focused during combat scenes.
SUMMARY
- Each player determines their Defense Bonus which is Armor Class minus 10
- Create an Attack Challenge Rating which is 11 + the attack modifier of the attacking monster
- The player rolls a d20 for Defense and adds their Defense Bonus
- The resulting roll is then compared against the Attack Challenge Rating
- A roll of a 1 by the player always hits and is a Critical Hit against the player
- A roll of 20 is always a dodge by the player – a critical miss for the monster
- A roll that meets or exceeds the Attack Challenge Rating is a miss
- A roll below the Attack Challenge Rating is a hit by the monster
THE EXAMPLE
Edward has an AC of 17 and is being attacked by an animated armour with +4 to hit.
Before the combat, you create Edwards DEFENCE BONUS by subtracting 10, giving you a +7. (17-10=7 defence bonus)
The DM has created a MONSTER CHALLENGE RATING by adding 11 to the hit bonus, giving a +15. (4+11=15 MCR)
DM: The monster turns to attack Edwards with a swift punch to the core, Edward make your defence roll.
You roll a D20 to add to your DEFENCE BONUS of +7. You roll an 11. 11+7 = 17. The DM lets you know that the monster did not land a blow and to proceed with the RP while they move onto the next attack.
Edward: Edward shifts to the right, dodging the mail fist aimed for him. (Say this versus a roll of 15 which would be a narrow miss or a 19 which would be an easy no sweat miss)
THE MATH
A creature normally gets a bonus to hit, so it would get an average of the d20, a 10 and its attack modifier would be added. So for the example above, it would give us 10+0 for a 10 CR. The player would roll a d20, and if he rolls a 1, then the monster gets past his defences. A 5% chance, and would in effect be a critical hit against the player.
We need to switch the player’s AC over to a defence bonus to their roll. Then create a Challenge Rating from the monster stats that the player is attempting to beat with their defence saving throw. And test all of that and make sure it works correctly. Since there is an underlying critical hit, that would be switch to a 1 on the roll instead of a 20.
Creating the Defense Bonus
I will take the player’s AC and subtract 10, and that is their Bonus on the Defense saving throw. Instead of a character having an AC of 17, then they would have a defence bonus of +7. On the other side, a character without armour and an average dexterity would have an AC of 10, so get a defence bonus of +0.Whenever anything attacked your player character you would then have to roll your defence saving throw with that defence bonus from your armour class.
Monster Attack Challenge Rating
A creature normally gets a bonus to hit, and then a random roll from a d20 to see if it hits the player.First, we take the average of the d20, which is a 10.5, and add the attack modifier. We get 11 + the attack modifier. This creates the Attack Challenge Rating. For a creature with a +5 to hit, that would be an Attack Challenge Rating of 16 (11+5). This is to correct for the fact that ties go to the die roller. But remember with this Defense system, a Roll of a 1 by the player always hits and is a critical hit, and a roll of 20 is always a dodge by the player (a critical miss).
Testing the Math Case #1 (Standard)
I’d like to use the same examples above and so we have a creature with a +5 to hit. Against a player wearing armour with an AC of 17.Player Rolling the die
So we create a defence bonus which would be a +7 for the player. The Monster Challenge Rating would be 16 (11+5). The player then needs to have a modified roll a 16 or above, or 11 or above on the unmodified roll. Which corresponds to a 9,10,11,12,14,15,16,17,18,19,20. That’s 11 out of 20 possibilities for a 55% chance of saving (dodging) against the monster’s attack. With a Roll of 1-9, the monster hits, and a 1 is a critical hit against the player.
Check that versus DM/Monster Rolling the die
On the other side of the coin, let's compare the math with standard attacking rolls. We would again have a creature with a +5 to hit. Against a player wearing armour with an AC of 17. If you were to roll a d20 for the monster, he would need to roll a 17 or greater to hit. So that is an 12,13,14,15,16,17,18,19,20 to make a hit against the player. That’s 9 out of 20 for a 45% chance to hit.
The odds remain exactly the same. 45% to hit versus 55% to dodge. The only thing that changes is that the players are making the rolls. So that seems to work.
Testing the Math Case #2 (Low Monster Level, High Player)
So let's look at a Low-level Monster that is trying to hit a player with a high AC. So we have a creature with a +4 to hit. Against a player wearing armour with an AC of 21.
Player Rolling the die
So we create a defence bonus which would be a +11 for the player. The Monster Challenge Rating would be 15 (11+4). The player then needs to have a modified roll of 15 or above or 4 or above on the unmodified roll. Which corresponds to a 4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20. That’s 17 out of 20 possibilities for an 85% chance of saving (dodging) against the monster’s attack. So with a Roll of 1-3 by the player, the monster hits, and a 1 is a critical hit against the player.
Testing the Math Case #3 (High Monster Level, Low Player)
So let's look at a High-level Monster that is trying to hit a player with a Low AC. So we have a creature with a +11 to hit. Against a player wearing armour with an AC of 10.
Player Rolling the die
We create a defence bonus which would be a +0 bonus for the player. The Monster Attack Challenge Rating would be 22 (11+11). The player then needs to have a modified roll of 22 or above. The monster will hit every time unless the player can roll a 20 to dodge the attack. So there is a 5% chance of saving (dodging) against the monster’s attack. And with a Roll of 1-19 by the player, the monster hits, and a 1 is a critical hit against the player. This is a pretty extreme case but is important to understand.