I’ve noodled about it before, but Wizards is noodling harder (the link goes to the enworld discussion. Wouldn’t you rather contextualize it, too?).
I think everyone can agree they kind of kacked this one up, because they ended up with a system that often rolls d20+ten million, with the ten million having been derived kind of humorously.
I think it’s fixable, though.
We’ll start at the smallest scale, and then zoom up.
The atom will be that two equally matched units have a 50/50 chance of killing the other (so a 25% chance they both wind up dead, and so forth).
For creatures with an integral challenge value, their bonus on this check is simply their challenge rating. Otherwise, it’s the number of steps below 1/2; in table form:
|Challenge Rating||Battle Rating|
|2 (and up)||+2 (and up)|
Each creature actively engaged with one or more enemies must make a death saving throw at the end of each battle round. Their bonus on this check is their own battle rating, and their DC is 10 + the average battle rating among their enemies.
If they fail, they take damage equal to the challenge rating of their foe (NOT the battle rating). If they’ve taken damage equal to their challenge rating, they perish.
Trying this out, a knight (CR 3; BR +3) is fighting a goblin (CR 1/4; BR -1). At the end of each battle round, the knight makes a DC 9 death save with a +3 bonus (25% chance of taking 1/4 damage), and the goblin a DC 13 death save with a -1 penalty (70% chance of taking 3 damage). That matches my intuition just fine: the goblin will be lucky to get a single hit off before being squelched, and is unlikely to seriously injure the knight, but it could happen.
Now let’s talk about numbers and scaling it up.
For a unit composed of multiple creatures, you derive a battle rating off of the average creature. In fact, you could figure out the CR of the average creature first, with a few provisos. The big one is that you shouldn’t figure out the CR of the average creature, but instead the CR of the average team. A person mounted on a horse simply adds the horse’s CR to their own. A shield wall has one spearman and one shield man working together. And so on. This is sort of like monster design; the gate to sum together CR instead of just averaging it is fuzzy but should at least include continuous physical contact while fighting: a berserker and a shield maiden probably don’t count, because they’re not glued to each other. Which makes the shield wall example a little forced, but you know what I mean.
So: if the death save is succeeded, the unit takes no damage.
If it is failed by <2, the unit takes 1% of the attacker’s remaining health as damage.
If it is failed by <5, the unit takes 5% of the attacker’s remaining health as damage.
Otherwise, it takes 25% of the attacker’s remaining health as damage.
Recalculate damage at the end of the round.