Activate Phyrexian Altar by sacrificing Undead Sprinter, adding {R}.
When Undead Sprinter dies, Pitiless Plunderer triggers, creating a Treasure token.
Activate the Treasure by tapping and sacrificing it, adding {B}.
Cast Undead Sprinter from your graveyard by paying {B}{R}.
Repeat.

Infinite creature LTB

Infinite creature ETB

Infinite creature sacrifice triggers

Infinite death triggers

Infinite storm count

Undead Sprinter

Pitiless Plunderer

Phyrexian Altar

Activate Phyrexian Altar by sacrificing Undead Sprinter, adding {B/R}.
Cast Undead Sprinter from your graveyard by paying {B}{R}.
Birgi triggers, adding {R}.
Activate Phyrexian Altar by sacrificing Undead Sprinter, adding {B}.
Repeat from step 2.

Birgi, God of Storytelling // Harnfel, Horn of Bounty

Activate Thermopod's second ability by sacrificing Undead Sprinter, adding {R}.
When Undead Sprinter dies, Pitiless Plunderer triggers, creating a Treasure token.
Activate the Treasure by tapping and sacrificing it, adding {B}.
Cast Undead Sprinter from your graveyard by paying {B}{R}.
Repeat.

Thermopod

Activate Phyrexian Altar by sacrificing Undead Sprinter, adding {B/R}.
If Runaway Steam-Kin has three or more +1/+1 counters on it, activate it by removing three +1/+1 counters from it, adding {R}{R}{R}.
Cast Undead Sprinter from your graveyard by paying {B}{R}.
Runaway Steam-Kin triggers, putting a +1/+1 counter on it.
Repeat from step 2.

Combos with "Undead Sprinter"