Elianar, Reborn Psychic

On each player's upkeep, Elianar, Reborn Psychic destroys target undead that player controls.
Whenever an undead dies, exile the top card of
their controller's library face down. You may 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.