Your devotion to black is at least ten or you have at least two opponents.
Steps
Activate Goblin Bombardment by sacrificing Gray Merchant, dealing 1 damage to any target.
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 damage.
Infinite death triggers.
Infinite ETB.
Infinite lifegain.
Infinite lifegain triggers.
Infinite lifeloss.
Infinite LTB.
Infinite sacrifice triggers.
Put all creature cards from all graveyards onto the battlefield under your control.