Inverted damage if undead fix poison bug help

Search assassin's C2 disassembly for "set element to poison" and you'll find the periodical damage routine. You need to expand and relocate (part of) it and add a bunch of conditional checks.

Honestly, I don't consider it a bug. It feels like the intended use of this property was to invert healing attacks on undead. I'd just remove it from periodical damage entirely and let Regen heal the undead and Seizure damage them. And remove the property from poison elemental attacks and let elemental absorption sort it out instead.