Activate Dimir House Guard by sacrificing Gray Merchant, regenerating it the next time it would die this turn.
Activate Chainer by paying nine life, returning Gray Merchant from your graveyard to the battlefield as a Nightmare in addition to its other creature types.
Gray Merchant enters the battlefield, causing each opponent to lose life equal to your devotion to black and causing you to gain life equal to the amount of life lost this way.
Repeat.

Infinite death triggers

Infinite ETB

Infinite lifegain

Infinite lifegain triggers

Infinite lifeloss

Infinite LTB

Infinite sacrifice triggers

Return all creature cards from all graveyards to the battlefield under your control

Chainer, Dementia Master

K'rrik, Son of Yawgmoth

Gray Merchant of Asphodel

Dimir House Guard

Activate Dimir House Guard by sacrificing it.
When Dimir House Guard dies, Luminous Broodmoth triggers, returning Dimir House Guard from your graveyard to the battlefield without a flying counter on it due to Solemnity.
Resolve the Dimir House Guard ability.
Repeat.

Solemnity

Luminous Broodmoth

Activate Dimir House Guard by sacrificing it.
When Dimir House Guard dies, its persist ability triggers, returning it from your graveyard to the battlefield without a -1/-1 counter on it due to Vizier of Remedies.
Resolve the Dimir House Guard ability.
Repeat.

Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight

Vizier of Remedies

Activate Dimir House Guard by sacrificing it.
When Dimir House Guard dies, Ratadrabik triggers, creating a token copy of Dimir House Guard that is a 2/2 legendary Zombie in addition to its other types.
Resolve the Dimir House Guard ability.
Repeat using the Dimir House Guard token

Ratadrabik of Urborg

Leyline of Singularity

Activate a basic Swamp by tapping it, adding {B}.
Activate Kenrith's fifth ability by paying {B} due to Agatha, returning Dimir House Guard from your graveyard to the battlefield.
Activate Earthcraft by tapping Dimir House Guard, untapping the basic Swamp.
Activate Dimir House Guard by sacrificing it.
Repeat.

Kenrith, the Returned King

Earthcraft

Agatha of the Vile Cauldron

Combos with "Dimir House Guard"