Valen, the Muted Storm

Reach, deathtouch, first strike, infect
A player leaving the game doesn't cause creatures
they own that you control to leave your control.
The first time Valen deals combat damage to a
player each turn, that player reveals all creature
cards from their hand. Exile one of those cards
face up. You may cast that spell and you can
spend mana as though it was mana of any color
to cast it.