Each player loses a third of their life, then discards a third of the cards in their hand, then sacrifices a third of the creatures they control, then sacrifices a third of the lands they control. Round up each time.

Dreadhorde Invasion

At the beginning of your upkeep, you lose 1 life and amass Zombies 1. (Put a +1/+1 counter on an Army you control. It's also a Zombie. If you don't control an Army, create a 0/0 black Zombie Army creature token first.)
Whenever a Zombie token you control with power 6 or greater attacks, it gains lifelink until end of turn.

Mogis, God of Slaughter

Indestructible
As long as your devotion to black and red is less than seven, Mogis isn't a creature.
At the beginning of each opponent's upkeep, Mogis deals 2 damage to that player unless they sacrifice a creature.

Altar's Reap

As an additional cost to cast this spell, sacrifice a creature.
Draw two cards.

Rakdos Guildgate

Rakdos Guildgate enters the battlefield tapped.
{T}: Add {B} or {R}.

Mayhem Devil

Whenever a player sacrifices a permanent, Mayhem Devil deals 1 damage to any target.

Zombify

Return target creature card from your graveyard to the battlefield.

Gilded Lotus

Cabal Stronghold

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

Kari Zev's Expertise

Gain control of target creature or Vehicle until end of turn. Untap it. It gains haste until end of turn.
You may cast a spell with mana value 2 or less from your hand without paying its mana cost.

Terminal Moraine

{T}: Add {C}.
{2}, {T}, Sacrifice Terminal Moraine: Search your library for a basic land card, put that card onto the battlefield tapped, then shuffle.

Blood Seeker

Whenever a creature enters the battlefield under an opponent's control, you may have that player lose 1 life.

Sadistic Glee

Enchant creature
Whenever a creature dies, put a +1/+1 counter on enchanted creature.

Swamp

Go for the Throat

Corpse Augur

When Corpse Augur dies, you draw X cards and you lose X life, where X is the number of creature cards in target player's graveyard.

Exquisite Blood

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

Underworld Connections

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

Word of Seizing

Split second (As long as this spell is on the stack, players can't cast spells or activate abilities that aren't mana abilities.)
Untap target permanent and gain control of it until end of turn. It gains haste until end of turn.

Thrill of Possibility

As an additional cost to cast this spell, discard a card.
Draw two cards.

Phyrexian Reclamation

{1}{B}, Pay 2 life: Return target creature card from your graveyard to your hand.

Juri, Master of the Revue

Whenever you sacrifice a permanent, put a +1/+1 counter on Juri, Master of the Revue.
When Juri dies, it deals damage equal to its power to any target.

Cradle to Grave

Destroy target nonblack creature that entered the battlefield this turn.

Daretti, Scrap Savant

+2: Discard up to two cards, then draw that many cards.
−2: Sacrifice an artifact. If you do, return target artifact card from your graveyard to the battlefield.
−10: You get an emblem with "Whenever an artifact is put into your graveyard from the battlefield, return that card to the battlefield at the beginning of the next end step."
Daretti, Scrap Savant can be your commander.

Ayara, Furnace Queen

At the beginning of combat on your turn, return up to one target artifact or creature card from your graveyard to the battlefield. It gains haste. Exile it at the beginning of the next end step.

Ayara, Widow of the Realm

{T}, Sacrifice another creature or artifact: Ayara, Widow of the Realm deals X damage to target opponent or battle and you gain X life, where X is the sacrificed permanent's mana value.
{5}{R/P}: Transform Ayara. Activate only as a sorcery. ({R/P} can be paid with either {R} or 2 life.)

Ayara, Widow of the Realm // Ayara, Furnace Queen

Syr Konrad, the Grim

Whenever another creature dies, or a creature card is put into a graveyard from anywhere other than the battlefield, or a creature card leaves your graveyard, Syr Konrad, the Grim deals 1 damage to each opponent.
{1}{B}: Each player mills a card. (They each put the top card of their library into their graveyard.)

Fleshbag Marauder

When Fleshbag Marauder enters the battlefield, each player sacrifices a creature.

Jet Medallion

Black spells you cast cost {1} less to cast.

Clackbridge Troll

Trample, haste
When Clackbridge Troll enters the battlefield, target opponent creates three 0/1 white Goat creature tokens.
At the beginning of combat on your turn, any opponent may sacrifice a creature. If a player does, tap Clackbridge Troll, you gain 3 life, and you draw a card.

Shadowblood Ridge

Sanguine Bond

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

Captivating Crew

{3}{R}: Gain control of target creature an opponent controls until end of turn. Untap that creature. It gains haste until end of turn. Activate only as a sorcery.

Deathgreeter

Whenever another creature dies, you may gain 1 life.

Phyrexian Arena

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

Vicious Offering

Kicker—Sacrifice a creature. (You may sacrifice a creature in addition to any other costs as you cast this spell.)
Target creature gets -2/-2 until end of turn. If this spell was kicked, that creature gets -5/-5 until end of turn instead.

Pilgrim's Eye

Flying
When Pilgrim's Eye enters the battlefield, you may search your library for a basic land card, reveal it, put it into your hand, then shuffle.

Tormenting Voice

Bond of Revival

Return target creature card from your graveyard to the battlefield. It gains haste until your next turn.

Feldon of the Third Path

{2}{R}, {T}: Create a token that's a copy of target creature card in your graveyard, except it's an artifact in addition to its other types. It gains haste. Sacrifice it at the beginning of the next end step.

Claim the Firstborn

Gain control of target creature with mana value 3 or less until end of turn. Untap that creature. It gains haste until end of turn.

Zulaport Cutthroat

Whenever Zulaport Cutthroat or another creature you control dies, each opponent loses 1 life and you gain 1 life.

Ribbons

Aftermath (Cast this spell only from your graveyard. Then exile it.)
Each opponent loses X life.

Cut // Ribbons

Mountain

Dark Petition

Search your library for a card, put that card into your hand, then shuffle.
Spell mastery — If there are two or more instant and/or sorcery cards in your graveyard, add {B}{B}{B}.

Evolving Wilds

{T}, Sacrifice Evolving Wilds: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Ancient Craving

You draw three cards and you lose 3 life.

Vraska's Contempt

Exile target creature or planeswalker. You gain 2 life.

Outpost Siege

As Outpost Siege enters the battlefield, choose Khans or Dragons.
• Khans — At the beginning of your upkeep, exile the top card of your library. Until end of turn, you may play that card.
• Dragons — Whenever a creature you control leaves the battlefield, Outpost Siege deals 1 damage to any target.

Meteor Golem

When Meteor Golem enters the battlefield, destroy target nonland permanent an opponent controls.

Rampaging Ferocidon

Menace
Players can't gain life.
Whenever another creature enters the battlefield, Rampaging Ferocidon deals 1 damage to that creature's controller.

Ravenous Chupacabra

When Ravenous Chupacabra enters the battlefield, destroy target creature an opponent controls.

Priest of the Blood Rite

When Priest of the Blood Rite enters the battlefield, create a 5/5 black Demon creature token with flying.
At the beginning of your upkeep, you lose 2 life.

Royal Assassin

Gravedigger

When Gravedigger enters the battlefield, you may return target creature card from your graveyard to your hand.

Gray Merchant of Asphodel

When Gray Merchant of Asphodel enters the battlefield, each opponent loses X life, where X is your devotion to black. You gain life equal to the life lost this way. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)

Disciple of Bolas

When Disciple of Bolas enters the battlefield, sacrifice another creature. You gain X life and draw X cards, where X is that creature's power.

Bolas's Citadel

You may look at the top card of your library any time.
You may play lands and cast spells from the top of your library. If you cast a spell this way, pay life equal to its mana value rather than pay its mana cost.
{T}, Sacrifice ten nonland permanents: Each opponent loses 10 life.

Ormendahl, Profane Prince

Westvale Abbey

{T}: Add {C}.
{5}, {T}, Pay 1 life: Create a 1/1 white and black Human Cleric creature token.
{5}, {T}, Sacrifice five creatures: Transform Westvale Abbey, then untap it.

Westvale Abbey // Ormendahl, Profane Prince

Marionette Master

Fabricate 3 (When this creature enters the battlefield, put three +1/+1 counters on it or create three 1/1 colorless Servo artifact creature tokens.)
Whenever an artifact you control is put into a graveyard from the battlefield, target opponent loses life equal to Marionette Master's power.

Final Parting

Search your library for two cards. Put one into your hand and the other into your graveyard. Then shuffle.

Blood Artist

Whenever Blood Artist or another creature dies, target player loses 1 life and you gain 1 life.

Noxious Gearhulk

Menace
When Noxious Gearhulk enters the battlefield, you may destroy another target creature. If a creature is destroyed this way, you gain life equal to its toughness.

Acidic Soil

Acidic Soil deals damage to each player equal to the number of lands they control.

Diabolic Tutor

Search your library for a card, put that card into your hand, then shuffle.

Ornithopter of Paradise

Whisper, Blood Liturgist

{T}, Sacrifice two creatures: Return target creature card from your graveyard to the battlefield.

Rise from the Grave

Put target creature card from a graveyard onto the battlefield under your control. That creature is a black Zombie in addition to its other colors and types.

Rekindling Phoenix

Flying
When Rekindling Phoenix dies, create a 0/1 red Elemental creature token with "At the beginning of your upkeep, sacrifice this creature and return target card named Rekindling Phoenix from your graveyard to the battlefield. It gains haste until end of turn."

Sign in Blood

Target player draws two cards and loses 2 life.

Painful Quandary

Whenever an opponent casts a spell, that player loses 5 life unless they discard a card.