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 creature ETB

Infinite creature LTB

Infinite creature sacrifice triggers

Nim Deathmantle

Ashnod's Altar

Headless Rider

Activate Relic of Legends by tapping Grimgrin, adding one mana.
Activate Grimgrin by sacrificing the Zombie castable for one mana, untapping Grimgrin and putting a +1/+1 counter on it.
When that creature dies, Headless Rider triggers, creating a creature token or a token that adds mana.
Activate Relic of Legends by tapping Grimgrin, adding one mana.
Activate Grimgrin by sacrificing the creature token, untapping Grimgrin and putting a +1/+1 counter on it.
Activate Havengul Lich by paying {1}, allowing you to cast the sacrificed Zombie from your graveyard this turn.
Cast that Zombie from your graveyard by paying one mana.
Repeat.

Infinite +1/+1 counters on a creature

Infinite storm count

Zombie Creature castable for one colored mana

Havengul Lich

Grimgrin, Corpse-Born

Relic of Legends

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

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"