"After the summoner dies, the undead do not vanish!"
That is to say, if Ye Huan finished off Spirit Slave, all the undead summoned by Spirit Slave would not disappear. Furthermore, because there was no summoner to control them, all the undead would regain their freedom. Aside from lacking physical bodies, they would be identical to normal people in every other aspect, such as having thoughts and consciousness!
However!
Chapter 838