tc;dr (too cheesy; didn't read). You lost me at 'breed with undead'.

There are easier, more elegant ways of achieving the desired result. For example, take Troll stats, add the Construct or Living Construct type, change trollish regeneration to 'fast repair' or something. Use. Don't tell the players these are reskinned trolls. Chortle evilly as they scrabble for workable tactics against these unstoppable killing machines.