Markash, the Crimson-Braided

Double strike, menace
Whenever Markash deals combat damage
to a player, that player may sacrifice a creature.
If they don't, create a treasure token and
exile the top card of that players library
face-down. You may look at that card and cast
it for as long as it remains exiled and you may
spend mana as though it were mana of any kind
to cast it.