Demons you control have menace, lifelink, and haste.
Starscourge — At the beginning of combat on your turn, exile up to one target creature card from a graveyard. If you exiled a card this way, create a token that's a copy of that card, except it's a 5/5 black Demon.
Ardyn is a mono-black reanimator-tribal hybrid: each combat you exile a creature card from any graveyard and stamp out a 5/5 Demon copy of it that swings immediately thanks to haste, lifelink, and menace. You ramp into Ardyn around turn 5-6 with rituals and mana rocks, fill graveyards (yours and opponents'), then grind out value Demon tokens every turn while attacking and gaining life. Demon payoffs and sacrifice synergies turn the tokens into both a beatdown and a value engine.
Free recursion every combat with no card spent, raiding any graveyard including opponents'
Tokens are full 5/5 Demons with the original's abilities plus evasion, lifelink, and haste — instant pressure
Mono-black gives access to the deck's best tutors, ramp, and recursion
Punishes opponents' creature decks by stealing their fatties as your Demons
Lifelink keeps you alive through aggressive racing
Eight mana to cast and the engine does nothing without Ardyn on the battlefield
Heavily commander-dependent; repeated removal/exile shuts the deck off