Activate Ashnod's Altar by sacrificing Headless Rider, adding {C}{C}.
When Headless Rider dies, it and Nim Deathmantle trigger,
Resolve the Headless Rider trigger, creating a creature token.
Holding priority, activate Ashnod's Altar by sacrificing a creature token, adding {C}{C}.
Resolve the Nim Deathmantle trigger, causing you to pay {4} to return Headless Rider from your graveyard to the battlefield and attach Nim Deathmantle to it.
Repeat.

Infinite death triggers

Infinite ETB

Infinite LTB

Infinite sacrifice triggers

Nim Deathmantle

Ashnod's Altar

Headless Rider

Cast Ghouls' Night Out by paying {3}{B}{B}, returning Eternal Witness from your graveyard to the battlefield, and at least two other creatures from your opponents' graveyards to the battlefield under your control.
When Eternal Witness enters the battlefield, it triggers, returning Ghouls' Night Out from your graveyard to your hand.
Activate Phyrexian Altar three times by sacrificing Eternal Witness and the two creatures your opponents own, adding {1}{B}{B}.
When the three creatures die, Headless Rider triggers three times, creating three 2/2 Zombie creature tokens.
Activate Phyrexian Altar twice by sacrificing two Zombies, adding {2}.
Repeat.

Infinite colored mana

Infinite creature tokens

Infinite magecraft triggers

Infinite storm count

Ghouls' Night Out

Phyrexian Altar

Eternal Witness

Cast Ghouls' Night Out by paying {3}{B}{B}, returning Salvager of Secrets from your graveyard to the battlefield, and at least two other creatures from your opponents' graveyards to the battlefield under your control.
When Salvager of Secrets enters the battlefield, it triggers, returning Ghouls' Night Out from your graveyard to your hand.
Activate Phyrexian Altar three times by sacrificing Salvager of Secrets and the two creatures your opponents own, adding {1}{B}{B}.
When the three creatures die, Headless Rider triggers three times, creating three 2/2 Zombie creature tokens.
Activate Phyrexian Altar twice by sacrificing two Zombies, adding {2}.
Repeat.

Salvager of Secrets

Cast Ghouls' Night Out by paying {3}{B}{B}, returning Mnemonic Wall from your graveyard to the battlefield, and at least two other creatures from your opponents' graveyards to the battlefield under your control.
When Mnemonic Wall enters the battlefield, it triggers, returning Ghouls' Night Out from your graveyard to your hand.
Activate Phyrexian Altar three times by sacrificing Mnemonic Wall and the two creatures your opponents own, adding {1}{B}{B}.
When the three creatures die, Headless Rider triggers three times, creating three 2/2 Zombie creature tokens.
Activate Phyrexian Altar twice by sacrificing two Zombies, adding {2}.
Repeat.

Mnemonic Wall

Cast Ghouls' Night Out by paying {3}{B}{B}, returning Shipwreck Dowser from your graveyard to the battlefield, and at least two other creatures from your opponents' graveyards to the battlefield under your control.
When Shipwreck Dowser enters the battlefield, it triggers, returning Ghouls' Night Out from your graveyard to your hand.
Activate Phyrexian Altar three times by sacrificing Shipwreck Dowser and the two creatures your opponents own, adding {1}{B}{B}.
When the three creatures die, Headless Rider triggers three times, creating three 2/2 Zombie creature tokens.
Activate Phyrexian Altar twice by sacrificing two Zombies, adding {2}.
Repeat.

Shipwreck Dowser

Cast Ghouls' Night Out by paying {3}{B}{B}, returning Archaeomancer from your graveyard to the battlefield, and at least two other creatures from your opponents' graveyards to the battlefield under your control.
When Archaeomancer enters the battlefield, it triggers, returning Ghouls' Night Out from your graveyard to your hand.
Activate Phyrexian Altar three times by sacrificing Archaeomancer and the two creatures your opponents own, adding {1}{B}{B}.
When the three creatures die, Headless Rider triggers three times, creating three 2/2 Zombie creature tokens.
Activate Phyrexian Altar twice by sacrificing two Zombies, adding {2}.
Repeat.

Combos with "Headless Rider"