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 creature ETB

Infinite creature LTB

Infinite creature sacrifice triggers

Nim Deathmantle

Ashnod's Altar

Iron League Steed

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.

Infinite scry 1

Eirdu, Carrier of Dawn // Isilu, Carrier of Twilight

Viscera Seer

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.

Combos with "Iron League Steed"