Activate Phyrexian Altar by sacrificing the one-drop creature, adding {B}.
When that creature dies, it and Blood Artist trigger.
Resolve the creature's ability, creating a creature token.
Resolve the Blood Artist ability, gaining at least one life and causing at least one opponent to lose at least one life.
When you gain life, Veinwitch Coven triggers, causing you to pay {B} to return the one-drop creature from your graveyard to your hand.
Activate Phyrexian Altar by sacrificing the token creature, adding one colored mana.
Cast the creature by paying one mana.
Repeat.

Infinite creature ETB

Infinite creature LTB

Infinite creature sacrifice triggers

Infinite death triggers

Infinite lifegain

One-drop creature with a second body

Veinwitch Coven

Blood Artist

Phyrexian Altar

Activate Phyrexian Altar by sacrificing the creature castable for {0}, adding {B}.
When that creature dies, Blood Artist triggers, causing you to gain 1 life and target player to lose 1 life.
When you gain life, Veinwitch Coven triggers, causing you to pay {B} to return the sacrificed creature from your graveyard to your hand.
Cast that creature by paying {0}.
Repeat.

Infinite lifegain triggers

Infinite lifeloss

Infinite storm count

Creature castable for {0} that survives

Activate Skirge Familiar by discarding a creature card, adding {B}.
Feast of Sanity triggers, dealing 1 damage to any target and causing you to gain 1 life.
Veinwitch Coven triggers, causing you to pay {B} to return any creature card from your graveyard to your hand.
Repeat.

Infinite damage

Infinite self-discard triggers

Feast of Sanity

Skirge Familiar

Cast the creature card from your hand for {0}. The creature enters, then immediately dies as a state-based action due to having 0 toughness.
When the creature enters, Carnival of Souls and Elas il-Kor trigger.
Resolve the Carnival of Souls ability, adding {B} and losing 1 life.
Resolve the Elas il-Kor ability, gaining at least 1 life.
When you gain life, Veinwitch Coven triggers, causing you to pay {B} to return the creature card from your graveyard to your hand.
Repeat.

Creature Castable for {0} that dies on enter

Carnival of Souls

Elas il-Kor, Sadistic Pilgrim

Activate Skirge Familiar by discarding Dryad Arbor, adding {B}.
Bone Miser's first and second ability triggers, along with Faith of the Devoted.
Resolve the first Bone Miser trigger, adding {B}{B}.
Resolve the second Bone Miser trigger, creating a 2/2 Zombie creature token.
Resolve the Faith of the Devoted trigger, causing you to pay {1} to cause each opponent to lose 2 life and you to gain two life.
Veinwitch Coven triggers, causing you to pay {B} to return Dryad Arbor from your graveyard to your hand.
Repeat.

Combos with "Veinwitch Coven"