Currently, nidbanes are completely and totally useless. They only KO their target at best, leaving some nidburns and disappearing. They aren't worth the mandrake root they cost considering how rare the roots are. If you send multiple nidbanes at a target and 1 KOs him, all the nidbanes will disappear. There isn't even a way to tell if your nidbane KO'd their target if you sent them, which is pretty shitty too.
If they KO their target, I think instead of disappearing, the nidbane should return to the dolmen it was sent from and be sent back out after a few hours wait time. Pretty much, they should hound their target until the nidbane is actually killed.