Xæl, Hellfire Archivist

This spell costs {1} less to cast for each creature card
in opponents' graveyards.

Vigilance, menace

Whenever Xæl enters the battlefield or attacks,
exile up to one creature card from each opponent’s graveyard. For each card exiled this way, create a token that’s a copy of it, except it’s a black Zombie
in addition to its other types and it gains haste.