Exdeath, Void Warlock's last ability checks your graveyard at the moment it would trigger to see if you have six or more permanent cards in your graveyard. If you don't, the ability won't trigger at all. If it does trigger, the ability will check again as it tries to resolve. If you don't have six or more permanent cards in your graveyard at that time, the ability won't resolve and none of its effects will happen.