Nefarox, Overlord of Grixis

Flying
Exalted (Whenever a creature you control attacks alone, that creature gets +1/+1 until end of turn.)
Whenever Nefarox, Overlord of Grixis attacks alone, defending player sacrifices a creature.

Underworld Connections

Enchant land
Enchanted land has "{T}, Pay 1 life: Draw a card."

Bontu's Monument

Black creature spells you cast cost {1} less to cast.
Whenever you cast a creature spell, each opponent loses 1 life and you gain 1 life.

Harvester of Souls

Deathtouch
Whenever another nontoken creature dies, you may draw a card.

Thraxodemon

{3}, {T}, Sacrifice another creature or artifact: Draw a card.

Duress

Target opponent reveals their hand. You choose a noncreature, nonland card from it. That player discards that card.

Defile

Target creature gets -1/-1 until end of turn for each Swamp you control.

Feed the Swarm

Destroy target creature or enchantment an opponent controls. You lose life equal to that permanent's mana value.

Profane Transfusion

Two target players exchange life totals. You create an X/X colorless Horror artifact creature token, where X is the difference between those players' life totals.

Doom Blade

Go for the Throat

Reaper from the Abyss

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

Hero's Downfall

Ob Nixilis, the Fallen

Landfall — Whenever a land enters the battlefield under your control, you may have target player lose 3 life. If you do, put three +1/+1 counters on Ob Nixilis, the Fallen.

Infernal Grasp

Destroy target creature. You lose 2 life.

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.

Basilisk Collar

Equipped creature has deathtouch and lifelink.
Equip {2}

Archfiend of Depravity

Flying
At the beginning of each opponent's end step, that player chooses up to two creatures they control, then sacrifices the rest.

Plague Drone

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

Overwhelming Remorse

This spell costs {1} less to cast for each creature card in your graveyard.
Exile target creature or planeswalker.

Sheoldred, Whispering One

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
At the beginning of your upkeep, return target creature card from your graveyard to the battlefield.
At the beginning of each opponent's upkeep, that player sacrifices a creature.

Songs of the Damned

Add {B} for each creature card in your graveyard.

K'rrik, Son of Yawgmoth

({B/P} can be paid with either {B} or 2 life.)
Lifelink
For each {B} in a cost, you may pay 2 life rather than pay that mana.
Whenever you cast a black spell, put a +1/+1 counter on K'rrik, Son of Yawgmoth.

Baleful Ammit

Lifelink
When Baleful Ammit enters the battlefield, put a -1/-1 counter on target creature you control.

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.)

Soul Shatter

Each opponent sacrifices a creature or planeswalker with the highest mana value among creatures and planeswalkers they control.

Cabal Stronghold

{T}: Add {C}.
{3}, {T}: Add {B} for each basic Swamp you control.

Read the Bones

Scry 2, then draw two cards. You lose 2 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.

Demolition Field

{T}: Add {C}.
{2}, {T}, Sacrifice Demolition Field: Destroy target nonbasic land an opponent controls. That land's controller may search their library for a basic land card, put it onto the battlefield, then shuffle. You may search your library for a basic land card, put it onto the battlefield, then shuffle.

Rune-Scarred Demon

Flying
When Rune-Scarred Demon enters the battlefield, search your library for a card, put it into your hand, then shuffle.

Charcoal Diamond

Charcoal Diamond enters the battlefield tapped.
{T}: Add {B}.

Abhorrent Overlord

Flying
When Abhorrent Overlord enters the battlefield, create a number of 1/1 black Harpy creature tokens with flying equal to your devotion to black. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)
At the beginning of your upkeep, sacrifice a creature.

Quicksilver Amulet

{4}, {T}: You may put a creature card from your hand onto the battlefield.

Pestilence Demon

Flying
{B}: Pestilence Demon deals 1 damage to each creature and each player.

Sanguine Bond

Whenever you gain life, target opponent loses that much life.

Murder

Bloodgift Demon

Flying
At the beginning of your upkeep, target player draws a card and loses 1 life.

Aspect of Gorgon

Enchant creature
Enchanted creature gets +1/+3 and has deathtouch. (Any amount of damage it deals to a creature is enough to destroy it.)

Demon of Loathing

Flying, trample
Whenever Demon of Loathing deals combat damage to a player, that player sacrifices a creature.

Sign in Blood

Target player draws two cards and loses 2 life.

Lord of the Void

Flying
Whenever Lord of the Void deals combat damage to a player, exile the top seven cards of that player's library, then put a creature card from among them onto the battlefield under your control.

Demon's Horn

Whenever a player casts a black spell, you may gain 1 life.

Archfiend of Spite

Flying
Whenever a source an opponent controls deals damage to Archfiend of Spite, that source's controller loses that much life unless they sacrifice that many permanents.
Madness {3}{B}{B} (If you discard this card, discard it into exile. When you do, cast it for its madness cost or put it into your graveyard.)

Demonic Embrace

Enchant creature
Enchanted creature gets +3/+1, has flying, and is a Demon in addition to its other types.
You may cast Demonic Embrace from your graveyard by paying 3 life and discarding a card in addition to paying its other costs.

Sower of Discord

Flying
As Sower of Discord enters the battlefield, choose two players.
Whenever damage is dealt to one of the chosen players, the other chosen player also loses that much life.

Indulgent Tormentor

Flying
At the beginning of your upkeep, draw a card unless target opponent sacrifices a creature or pays 3 life.

Syphon Mind

Each other player discards a card. You draw a card for each card discarded this way.

Swamp

Phyrexian Obliterator

Trample
Whenever a source deals damage to Phyrexian Obliterator, that source's controller sacrifices that many permanents.

Ob Nixilis, Unshackled

Flying, trample
Whenever an opponent searches their library, that player sacrifices a creature and loses 10 life.
Whenever another creature dies, put a +1/+1 counter on Ob Nixilis, Unshackled.

Vraska's Contempt

Exile target creature or planeswalker. You gain 2 life.

Inquisition of Kozilek

Target player reveals their hand. You choose a nonland card from it with mana value 3 or less. That player discards that card.

In Garruk's Wake

Destroy all creatures you don't control and all planeswalkers you don't control.

Whispersilk Cloak

Equipped creature can't be blocked and has shroud. (It can't be the target of spells or abilities.)
Equip {2}

Lord of the Forsaken

Flying, trample
{B}, Sacrifice another creature: Target player mills three cards.
Pay 1 life: Add {C}. Spend this mana only to cast a spell from your graveyard.

Alchemist's Gift

Target creature gets +1/+1 and gains your choice of deathtouch or lifelink until end of turn. (Any amount of damage a creature with deathtouch deals to a creature is enough to destroy it. Damage dealt by a creature with lifelink also causes its controller to gain that much life.)

Withengar Unbound

Flying, intimidate, trample (A creature with intimidate can't be blocked except by artifact creatures and/or creatures that share a color with it.)
Whenever a player loses the game, put thirteen +1/+1 counters on Withengar Unbound.

Elbrus, the Binding Blade

Equipped creature gets +1/+0.
When equipped creature deals combat damage to a player, unattach Elbrus, the Binding Blade, then transform it.
Equip {1}

Elbrus, the Binding Blade // Withengar Unbound

Slaughter Pact

Destroy target nonblack creature.
At the beginning of your next upkeep, pay {2}{B}. If you don't, you lose the game.

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.

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.

Dreadhound

When Dreadhound enters the battlefield, mill three cards. (Put the top three cards of your library into your graveyard.)
Whenever a creature dies or a creature card is put into a graveyard from a library, each opponent loses 1 life.

Agent of the Shadow Thieves

Commander creatures you own have "Whenever this creature attacks a player, if no opponent has more life than that player, put a +1/+1 counter on this creature. It gains deathtouch and indestructible until end of turn."

Vat Emergence

Put target creature card from a graveyard onto the battlefield under your control. Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Corrupt

Corrupt deals damage to any target equal to the number of Swamps you control. You gain life equal to the damage dealt this way.

Night's Whisper

Helm of Awakening

Liliana's Contract

When Liliana's Contract enters the battlefield, you draw four cards and you lose 4 life.
At the beginning of your upkeep, if you control four or more Demons with different names, you win the game.

Taborax, Hope's Demise

Flying
Taborax, Hope's Demise has lifelink as long as it has five or more +1/+1 counters on it.
Whenever another nontoken creature you control dies, put a +1/+1 counter on Taborax. If that creature was a Cleric, you may draw a card. If you do, you lose 1 life.

Rakshasa Debaser

Whenever Rakshasa Debaser attacks, put target creature card from defending player's graveyard onto the battlefield under your control.
Encore {6}{B}{B} ({6}{B}{B}, Exile this card from your graveyard: For each opponent, create a token copy that attacks that opponent this turn if able. They gain haste. Sacrifice them at the beginning of the next end step. Activate only as a sorcery.)

Phyrexian Arena

At the beginning of your upkeep, you draw a card and you lose 1 life.

Promise of Power

Choose one —
• You draw five cards and you lose 5 life.
• Create an X/X black Demon creature token with flying, where X is the number of cards in your hand.
Entwine {4} (Choose both if you pay the entwine cost.)

Exsanguinate

Each opponent loses X life. You gain life equal to the life lost this way.

Elixir of Immortality

{2}, {T}: You gain 5 life. Shuffle Elixir of Immortality and your graveyard into their owner's library.

Doom Whisperer

Flying, trample
Pay 2 life: Surveil 2. (Look at the top two cards of your library, then put any number of them into your graveyard and the rest on top of your library in any order.)

Sheoldred's Edict

Choose one —
• Each opponent sacrifices a nontoken creature.
• Each opponent sacrifices a creature token.
• Each opponent sacrifices a planeswalker.

Demonlord Belzenlok

Flying, trample
When Demonlord Belzenlok enters the battlefield, 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.

Mirrodin Avenged

Destroy target creature that was dealt damage this turn.
Draw a card.

Nightmare Shepherd

Flying
Whenever another nontoken creature you control dies, you may exile it. If you do, create a token that's a copy of that creature, except it's 1/1 and it's a Nightmare in addition to its other types.

Dismember

({B/P} can be paid with either {B} or 2 life.)
Target creature gets -5/-5 until end of turn.

Embodiment of Agonies

Flying, deathtouch
Embodiment of Agonies enters the battlefield with a +1/+1 counter on it for each different mana cost among nonland cards in your graveyard. (For example, {2}{B} and {1}{B}{B} are different mana costs.)

Dark Ritual

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Vilis, Broker of Blood

Flying
{B}, Pay 2 life: Target creature gets -1/-1 until end of turn.
Whenever you lose life, draw that many cards. (Damage causes loss of life.)