At the beginning of each player's end step, if no permanents left the battlefield this turn, that player loses X life, where X is Sarevok's power. Choose a Background (You can have a Background as a second commander.)

{T}: Add one mana of any color in your commander's color identity.

When Gray Merchant of Asphodel enters the battlefield, each opponent loses X life, where X is your devotion to black. You gain life equal to the life lost this way. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)

Add {B}{B}{B}.