Elianar, Reborn Psychic

On each player's upkeep, Elianar destroys target
undead creature that player controls. (Undead creatures are zombies, skeletons, vampires, and spirits)
Whenever an undead dies, exile the top card of
their controller's library face down. You may
look at and play cards exiled this way for as long
as they remain exiled, and you may spend mana
as if it were mana of any color to cast those spells.