Im playing a modpack with this mod installed and a mob called Zach (Skeleton) never dies, when he is on 1 of HP and i try to do the last hit this doesn't deal any damage and a sound like a thunder is triggered, then the floor near to this mob spawns a low layer of snow, i tried to hit without any weapon and the lowest amount of HP with wich i manage to leave is 0.25 of HP, this mob is unkillable or i don't know how to kill this if it's a normal tipe of mob.

Likely has some enchantment from some mod making him unkillable, it happens from time to time. Nothing I can do from my side.