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.

Malfegor

Flying
When Malfegor enters, discard your hand. Each opponent sacrifices a creature of their choice for each card discarded this way.

Rakdos, the Showstopper

Flying, trample
When Rakdos enters, flip a coin for each creature that isn't a Demon, Devil, or Imp. Destroy each creature whose coin comes up tails.

Great Unclean One

Reverberating Summons — At the beginning of your end step, each opponent loses 2 life. Then for each opponent who has less life than you, create a 1/3 black Demon creature token named Plaguebearer of Nurgle.

Herald of Slaanesh

Locus of Slaanesh — Demon spells you cast cost {2} less to cast.
Other Demons you control have haste.

Raise Dead

Return target creature card from your graveyard to your hand.

Have

Electrickery

Electrickery deals 1 damage to target creature you don't control.
Overload {1}{R} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

Swamp

Jaya's Greeting

Jaya's Greeting deals 3 damage to target creature. Scry 1.

Plague Drone

Flying
Rot Fly — If an opponent would gain life, that player loses that much life instead.

Demon Bolt

Demon Bolt deals 4 damage to target creature or planeswalker.
Foretell {R} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)

Harvester of Souls

Deathtouch (Any amount of damage this deals to a creature is enough to destroy it.)
Whenever another nontoken creature dies, you may draw a card.

Heartfire

As an additional cost to cast this spell, sacrifice a creature or planeswalker.
Heartfire deals 4 damage to any target.

Orcus, Prince of Undeath

Flying, trample
When Orcus enters, choose one —
• Each other creature gets -X/-X until end of turn. You lose X life.
• Return up to X target creature cards with total mana value X or less from your graveyard to the battlefield. They gain haste until end of turn.

Mountain

Orca, Siege Demon

Trample
Whenever another creature dies, put a +1/+1 counter on Orca.
When Orca dies, it deals damage equal to its power divided as you choose among any number of targets.

Anger

Haste
As long as this card is in your graveyard and you control a Mountain, creatures you control have haste.

Unfortunate Accident

Spree (Choose one or more additional costs.)
+ {2}{B} — Destroy target creature.
+ {1} — Create a 1/1 red Mercenary creature token with "{T}: Target creature you control gets +1/+0 until end of turn. Activate only as a sorcery."

Reaper from the Abyss

Flying
Morbid — At the beginning of each end step, if a creature died this turn, destroy target non-Demon creature.

Lightning Bolt

Lightning Bolt deals 3 damage to any target.

Terminate

Destroy target creature. It can't be regenerated.

Tramway Station

This land enters tapped.
{T}: Add {B} or {R}.
{2}{B}{R}, {T}, Sacrifice this land: Draw a card.

Ob Nixilis, the Adversary

Casualty X. The copy isn't legendary and has starting loyalty X. (As you cast this spell, you may sacrifice a creature with power X. When you do, copy this spell. The copy becomes a token.)
+1: Each opponent loses 2 life unless they discard a card. If you control a Demon or Devil, you gain 2 life.
−2: Create a 1/1 red Devil creature token with "When this token dies, it deals 1 damage to any target."
−7: Target player draws seven cards and loses 7 life.

Kothophed, Soul Hoarder

Flying
Whenever a permanent owned by another player is put into a graveyard from the battlefield, you draw a card and you lose 1 life.

Black Market

Whenever a creature dies, put a charge counter on this enchantment.
At the beginning of your first main phase, add {B} for each charge counter on this enchantment.