Which data file(s) determine whether an NPC is immortal or not?

Back when used to play Skyrim (it’s been awhile), I would use a mod to strip certain NPCs of their immortality. Mainly because I found them to be annoying.

For KCD, I don’t plan to kill off any essential characters (yet). Perhaps when I’ve finished the game I’ll try to figure it out.

However, I’m pretty sure the values that determine whether an NPC is killable, is almost certainly present in the game’s data files. Can someone say which file holds these values, and exactly what to edit?