Sienna, the Song of Dusk

At the beginning of your upkeep, look at the top
X cards of target opponent's library, where X is your devotion to black. Exile one of them, then put the rest on the bottom of that library in a random order. You may cast that card for as long as it remains exiled, and you may spend black mana as though it were mana of any type to cast that spell.
Pay 2 life: Add {b}. Spend this mana only to cast spells exiled by Sienna.