Level Up (A5E) Creating Monsters: Two Questions

[Tessarael]

Another thing to account for is the very high save DCs for high CR creatures. As an example, Ancient Red Dragon (CR 26) breath weapon has a Dexterity save DC 25 for half. The Rogue with high Dexterity and Dexterity saving throw proficiency may have a 50% chance of making that save for zero damage, half on failure, assuming they have Evasion. If your other party members are say Cleric, Fighter, and Wizard, they don't have Dexterity saving throw proficiency and will fail that saving throw. Maybe one of them has Fire Resistance or a power like the Shield Focus feat to improve Dexterity saves and take reduced damage on a success.

I'd be inclined to be conservative and assume that a 90' breath weapon cone will hit 3 or 4 of the party members, and do on average 75% damage given the very high save DC (Rogue avoiding it, others not). So in that case, the damage multiplier on that breath weapon is between 2x and 3x.

I also think that there is a lack of accurate probability statistics going into the "5-6 = 1 round, 3-4 = 2 rounds, 1-2 = 3 rounds" recharge rate calculation:

With a recharge (5-6) for a breath weapon, the dragon breathes on round 1. Second round, 1/3 chance they can breathe again. Third round, 1/3 chance they can breath again. So by the end of round 3, assuming the dragon breathes fire as much as they can, the expected average number of breaths is 1+1/3+1/3 = 5/3. It is not just 1 round out of 3.

With a recharge (4-6), the power is used round 1, 50% chance round 2, and 50% chance round 3. So average is 2 out of 3 rounds.

 

[TheDungeonMathster]

I recall doing an analysis a while back where, on a CR vs equivalently leveled party basis, the probability to hit a PC was 68% (on average) and the probability for a PC to make a saving throw was roughly the same. Thus, if you have a monster that does d damage per round with standard melee or ranged attacks, their expected damage is actually (0.68)d.

Likewise, if you have a creature that deals d damage per round via spells, but the spells are "save for half," then the expected damage is (0.68)(d/2) + (0.32)d = (0.34)d + (0.32)d = 0.66d. (68% of the time, you deal half damage and 32% of the time you deal full damage).

The difference is negligible in the end.

However, there are spells that are "save for no damage." In these cases, if the spell deals d damage, then the expected damage should be (0.32)d + (0.68)0 = 0.32d.

This means that you double the damage output from "save for no damage" spells when placing them in your monster's arsenal.

@Tessarael Huh, I never thought about that, but you're totally right. If they wanted it to work as once per three rounds, it should not be "Recharge," but "Roll to Use."