Demon's Disciple

When Demon's Disciple enters the battlefield, each player sacrifices a creature or planeswalker.

Raving Oni-Slave

When Raving Oni-Slave enters or leaves the battlefield, you lose 3 life if you don't control a Demon.

Ob Nixilis of the Black Oath

+2: Each opponent loses 1 life. You gain life equal to the life lost this way.
−2: Create a 5/5 black Demon creature token with flying. You lose 2 life.
−8: You get an emblem with "{1}{B}, Sacrifice a creature: You gain X life and draw X cards, where X is the sacrificed creature's power."
Ob Nixilis of the Black Oath can be your commander.

Blade of the Oni

Menace
Equipped creature has base power and toughness 5/5, has menace, and is a black Demon in addition to its other colors and types.
Reconfigure {2}{B}{B} ({2}{B}{B}: Attach to target creature you control; or unattach from a creature. Reconfigure only as a sorcery. While attached, this isn't a creature.)

Skirsdag High Priest

Morbid — {T}, Tap two untapped creatures you control: Create a 5/5 black Demon creature token with flying. Activate only if a creature died this turn.

Reiver Demon

Flying
When Reiver Demon enters the battlefield, if you cast it from your hand, destroy all nonartifact, nonblack creatures. They can't be regenerated.

Apocalypse Demon

Flying
Apocalypse Demon's power and toughness are each equal to the number of cards in your graveyard.
At the beginning of your upkeep, tap Apocalypse Demon unless you sacrifice another creature.

Blood Speaker

At the beginning of your upkeep, you may sacrifice Blood Speaker. If you do, search your library for a Demon card, reveal that card, put it into your hand, then shuffle.
Whenever a Demon enters the battlefield under your control, return Blood Speaker from your graveyard to your hand.

Abomination of Llanowar

Vigilance; menace (This creature can't be blocked except by two or more creatures.)
Abomination of Llanowar's power and toughness are each equal to the number of Elves you control plus the number of Elf cards in your graveyard.