Your devotion to black is at least ten or you have at least two opponents.
Steps
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.
Results
Infinite death triggers.
Infinite creature ETB.
Infinite lifegain.
Infinite lifegain triggers.
Infinite lifeloss.
Infinite creature LTB.
Infinite creature sacrifice triggers.
Return all creature cards from all graveyards to the battlefield under your control.
Metadata
Bracket tag
ExhibitionBracket 1+
For any deck
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1 · Exhibition5 · cEDH
Bracket tags are estimated from the cards and results of each combo, as a guideline rather than a strict classification.