Second Coming of Isha

Undying
Whenever an opponent casts a spell, create
X 1/1 black Drudge Skeleton tokens with
"{b}:Regenerate Drudge Skeleton", where X
is that spells converted mana cost.
Sacrifice X creatures: search your library for
a black card with converted mana cost X, reveal it, put it into your hand, and shuffle
your library.