When Demonlord Belzenlok enters, exile cards from the top of your library until you exile a nonland card, then put that card into your hand. If the card's mana value is 4 or greater, repeat this process. Demonlord Belzenlok deals 1 damage to you for each card put into your hand this way.
Cast Belzenlok as a 6/6 flying, trampling beater that refills your hand on arrival, then leverage mono-black's reanimation, drain, and recursion to grind opponents out. Curve into the commander around turn 5-6, draw big chunks of your deck loaded with high-cost bombs, and convert that card advantage into a lethal attacker or an aristocrats/drain engine.
Strong card advantage stapled to a 6/6 flier with evasion that pressures life totals immediately
Mono-black gives access to the format's best tutors, removal, and reanimation
Rewards a high-cost curve, so its draw can dig several cards deep at once
Easy to recur and re-cast for repeated card refills with sacrifice and reanimation effects
The ETB deals damage to you for each card, which can be punishing or even lethal with a top-heavy deck
Mono-black struggles with enchantment and artifact removal and lacks card draw outside life payment
No built-in protection; a removal spell wastes your six mana and the trigger value
Color-screwed against fast combo and flyer-light boards that race you down
Converts your heavy black devotion into a massive drain that can close games.
Lean into life-gain to offset the self-damage—Sheoldred, the Apocalypse, Beseech the Mirror, and Vito enable aggressive card draw without killing yourself. Add fast mana (Dark Ritual, Jet Medallion, Cabal Coffers + Urborg) and efficient tutors (Demonic Tutor, Vampiric Tutor, Imperial Seal) to assemble drain or reanimation combos faster. Tighten the curve so the ETB damage stays manageable, and include cheap reanimation plus sac outlets to loop Belzenlok for repeated value.