Activate Ashnod's Altar by sacrificing Iron League Steed, adding {C}{C}.
When Iron League Steed dies, Nim Deathmantle triggers, causing you to pay {4} to return Iron League Steed from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Iron League Steed enters the battlefield, its fabricate ability triggers, creating a 1/1 Servo artifact creature token.
Activate Ashnod's Altar by sacrificing the Servo, adding {C}{C}.
Repeat.

Infinite death triggers

Infinite ETB

Infinite LTB

Infinite sacrifice triggers

Nim Deathmantle

Ashnod's Altar

Iron League Steed

Activate Krark-Clan Ironworks by sacrificing Iron League Steed, adding {C}{C}.
When Iron League Steed dies, Nim Deathmantle triggers, causing you to pay {4} to return Iron League Steed from your graveyard to the battlefield, and attach Nim Deathmantle to it.
When Iron League Steed enters the battlefield, its fabricate ability triggers, creating a 1/1 Servo artifact creature token.
Activate Krark-Clan Ironworks by sacrificing the Servo, adding {C}{C}.
Repeat.

Krark-Clan Ironworks

Activate Viscera Seer by sacrificing Iron League Steed.
When Iron League Steed dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
When Iron League Steed enters, its fabricate ability triggers, putting a +1/+1 counter on it.
Resolve the Viscera Seer ability.
Repeat.

Combos with "Iron League Steed"