Dark Necromancer

When Dark Necromancer enters the battlefield
and at the beginning of your upkeep, you may
pay {u/b}{u/b}, if you do, create a 1/1 black Skeleton Creature token.
{b}{b}{t}: Regenerate target creature.
At the beginning of each end step, if you don't
control a Skeleton, sacrafice Dark Necromancer.