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.)
Sarevok punishes inaction: each turn nobody sacrifices, destroys, or loses a permanent, every player drains for Sarevok's power, so you want a big, hard-to-interact-with Sarevok while you grind the board. Pair him with a Background that scales his power (or supports aggression), play few-trigger turns where the board stays static, and pile on counters to turn the end-step drain into a clock that kills the whole table at once.
Free, repeatable table-wide drain that doesn't require attacking or combat
Background slot gives huge deckbuilding flexibility and a built-in second commander
Mono-black access to tutors, reanimation, and powerful card advantage
Scales hard with +1/+1 counters and power-pumping effects into a real clock
The 'no permanents left the battlefield' clause is easily disabled by token decks, fetchlands, and any sacrifice/removal effect
Sarevok at base 3 power is a fragile must-kill threat that draws removal
Mono-black has poor enchantment/artifact removal and no card-draw-to-graveyard recursion of blue
Drain hits you too if you can't keep your own permanents intact
Legendary Creature — Human Knight
