Very powerful, but considering the constant damage you take from heat (that can't be resisted) I think this is alright. There's already a way to make shock or ice or other element types super strong, and even though fire has some good spells it falls behind if you're trying to build for it exclusively.
I believe so yes. It may be a bug, but I tried it myself with item granted resistance and some other users noticed that tieflings still take the full amount of damage with their intrinsic resistances. If fire resistance worked it may not have been so bad in the base game, just not great. An alternate buff to the heat mechanic would probably involve some attack roll buffs and reducing heat damage taken to 1 or something, but honestly this is fine. There are so many other elemental specializations, fire-build items are kinda meh, there are so many fire resistant enemies, and eldritch blast can be busted way harder than this anyway.
4 comments