At the moment, the Dark Pact skill is great for creating different variations of possessed but at the cost of eliminating mutants as a permanent troop. At present, there’s really no good reason to keep a warrior as a mutant when you could just keep going on to possession. The prospect of risking death or insanity isn’t a deterrent because, ironically, there’s just so much death and insanity already; it all runs together in a single slog. My suggestion is this: change the Dark Pact skill so that a percentage of possessions fail and, instead, add an extra arm while also locking the skill tree. I won’t go into it here (unless asked) but there are various ways to add an extra arm that won’t require any modification to armor. If these three-armed mutants prove too powerful, then the extra arm could also be balanced out with a minor negative side effect.