Gizahal, Mind Reaper

Flying
If Gizahal, Mind Reaper would become the target
of a spell or ability, sacrifice it.
When Gizahal, Mind Reaper is put into a
graveyard, you may discard a card. If you do,
return Gizahal, Mind Reaper to your hand.
If another Nightmare creature you control would
be sacrificed, put it on top of your library instead.