Activate Yawgmoth by paying 1 life and sacrificing the undying creature.
When the creature dies, Deathgreeter, Pawn of Ulamog and the undying creature's undying ability trigger.
Resolve the Deathgreeter trigger, causing you to gain 1 life.
Resolve the Pawn of Ulamog ability, creating a 0/1 Eldrazi Spawn creature token.
Resolve the undying trigger, returning the creature from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the Yawgmoth ability, putting a -1/-1 counter on any creature and drawing a card.
Activate Yawgmoth by paying 1 life and sacrificing another creature.
When the creature dies, Deathgreeter triggers, causing you to gain 1 life 
Resolve the Yawgmoth ability, putting a -1/-1 counter on the undying creature and drawing a card.
Repeat.

Infinite card draw

Infinite draw triggers

Near-infinite creature ETB

Near-infinite creature LTB

Near-infinite creature sacrifice triggers

Near-infinite death triggers

Near-infinite lifegain triggers

Creature with Undying

Yawgmoth, Thran Physician

Deathgreeter

Pawn of Ulamog

Activate Warren Soultrader by paying 1 life and sacrificing Gravecrawler.
When Gravecrawler dies, Deathgreeter triggers, causing you to gain 1 life and target opponent to lose 1 life.
Resolve the Warren Soultrader ability, creating a Treasure token.
Activate the Treasure by tapping and sacrificing it, adding {B}.
Cast Gravecrawler from your graveyard by paying {B}.
Repeat.

Infinite creature ETB

Infinite creature LTB

Infinite creature sacrifice triggers

Infinite death triggers

Infinite lifegain triggers

Infinite storm count

Warren Soultrader

Gravecrawler

Activate Warren Soultrader by paying 1 life and sacrificing Reassembling Skeleton.
When Reassembling Skeleton dies, Deathgreeter and Pitiless Plunderer trigger.
Resolve the Deathgreeter trigger, causing you to gain 1 life.
Resolve the Pitiless Plunderer trigger, creating a Treasure token.
Resolve the Warren Soultrader ability, creating a Treasure token.
Activate the Treasures by tapping and sacrificing them, adding {1}{B}.
Activate Reassembling Skeleton by paying {1}{B}, returning it from your graveyard to the battlefield.
Repeat.

Infinite artifact ETB

Infinite artifact LTB

Infinite artifact sacrifice triggers

Pitiless Plunderer

Reassembling Skeleton

Activate Carrion Feeder by sacrificing Gravecrawler.
Deathgreeter triggers, causing you to gain 1 life.
Resolve the Carrion Feeder ability, putting a +1/+1 counter on it.
Cast Gravecrawler from your graveyard by paying 2 life.
Gravecrawler enters the battlefield, triggering Ayara, causing each opponent to lose 1 life and you to gain 1 life.
Repeat.

Infinite +1/+1 counters on a creature

Infinite lifeloss

K'rrik, Son of Yawgmoth

Ayara, First of Locthwain

Carrion Feeder

Cast God-Eternal Bontu either from your hand or from the command zone by paying {3}{B}{B} and commander tax, if applicable.
Endrek Sahr triggers, creating five 1/1 Thrull creature tokens.
The Thrull tokens enter the battlefield, triggering Carnival of Souls five times.
Activate Phyrexian Altar by sacrificing two Thrull tokens, adding {B}{B}.
Deathgreeter triggers twice, causing you to gain two life.
Resolve each Carnival of Souls, causing you to lose five life and add {B}{B}{B}{B}{B}.
Resolve God-Eternal Bontu from step 1.
God-Eternal Bontu enters the battlefield, triggering its first ability.
Holding priority, activate Phyrexian Altar by sacrificing God-Eternal Bontu, adding {B}.
God-Eternal Bontu's second ability and Deathgreeter trigger.
Resolve the God-Eternal Bontu trigger, causing you to put it into your library third from the top from your graveyard.
Resolve the Deathgreeter trigger, causing you to gain 1 life.
Resolve God-Eternal Bontu's first ability from step 8, sacrificing three Thrulls and drawing three cards.
Deathgreeter triggers three times, causing you to gain three life.
Repeat.
Once you have an empty library, you can continue the combo by only sacrificing one Thrull in step 13, and sacrificing the other two Thrulls to Phyrexian Altar.

Infinite colored mana

Infinite lifegain

God-Eternal Bontu

Endrek Sahr, Master Breeder

Carnival of Souls

Phyrexian Altar

Activate Chainer by paying {B}{B}{B} and 3 life, returning Grave Titan from your graveyard to the battlefield.
When Grave Titan enters, it triggers, creating two 2/2 Zombie creature tokens.
Activate Phyrexian Altar three times by sacrificing Grave Titan and the two Zombies, adding {B}{B}{B}
When Grave Titan and the Zombie tokens die, Deathgreeter triggers three times, causing you to gain 1 life for each trigger.
Repeat.

Chainer, Dementia Master

Grave Titan

Activate Unfulfilled Desires by paying {1} and 1 life, causing you to loot.
The Locust God triggers, creating a 1/1 Insect creature token with flying and haste.
Activate Ashnod's Altar by sacrificing the Insect, adding {C}{C}
When the Insect dies, Deathgreeter triggers, causing you to gain 1 life.
Repeat.

Combos with "Deathgreeter"