Activate Pyreheart Wolf using Walking Ballista's second ability by removing a +1/+1 counter from it.
Repeat step 1 until Pyreheart Wolf has no +1/+1 counters on it.
Resolve all Pyreheart Wolf abilities dealing 1 damage to Pyreheart Wolf, and any remaining damage to any target(s).
Pyreheart Wolf dies as a state-based action due to having 0 toughness.
Pyreheart Wolf's undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Repeat.

Infinite ETB

Infinite LTB

Infinite death triggers

Agatha's Soul Cauldron

Walking Ballista

Pyreheart Wolf

Activate Pyreheart Wolf using Triskelion's ability by removing a +1/+1 counter from it.
Repeat step 1 until Pyreheart Wolf has no +1/+1 counters on it.
Resolve all Pyreheart Wolf abilities dealing 1 damage to Pyreheart Wolf, and any remaining damage to any target(s).
Pyreheart Wolf dies as a state-based action due to having 0 toughness.
Pyreheart Wolf's undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Repeat.

Triskelion

Activate Ashnod's Altar by sacrificing Pyreheart Wolf, adding {C}{C}.
When Pyreheart Wolf dies, its undying ability and Sengir's first ability trigger.
Resolve the Pyreheart Wolf trigger, returning it from your graveyard to the battlefield with a +1/+1 counter and a vigilance counter on it.
Resolve the Sengir trigger, putting two +1/+1 counters on Sengir.
Activate Tayam by paying {3}, removing a +1/+1 counter from Sengir, and both counters from Pyreheart Wolf.
Holding priority, activate Ashnod's Altar by sacrificing Pyreheart Wolf, adding {C}{C}.
Repeat steps 2-4 one additional time.
Activate Ashnod's Altar by sacrificing Pyreheart Wolf, adding {C}{C}.
Resolve Tayam's ability from step 5, causing you to mill three cards and returning Pyreheart Wolf from your graveyard to the battlefield.
Repeat.

Infinite +1/+1 counters on a creature

Infinite colorless mana

Infinite sacrifice triggers

Infinite self-mill

Put a selection of permanant cards from your library and graveyard onto the battlefield

Tayam, Luminous Enigma

Sengir, the Dark Baron

Ashnod's Altar

Activate Ashnod's Altar by sacrificing Pyreheart Wolf, adding {C}{C}.
When Pyreheart Wolf dies, its undying ability and Necrosynthesis trigger.
Resolve the Pyreheart Wolf trigger, returning it from your graveyard to the battlefield with a +1/+1 counter and a vigilance counter on it.
Resolve the Necrosynthesis trigger, putting a +1/+1 counter on Tayam.
Activate Tayam by paying {3}, removing a +1/+1 counter from Tayam, and both counters from Pyreheart Wolf.
Holding priority, activate Ashnod's Altar by sacrificing Pyreheart Wolf, adding {C}{C}.
Repeat steps 2-4 one additional time.
Activate Ashnod's Altar by sacrificing Pyreheart Wolf, adding {C}{C}.
Resolve Tayam's ability from step 5, causing you to mill three cards and returning Pyreheart Wolf from your graveyard to the battlefield.
Repeat.

Necrosynthesis

Combos with "Pyreheart Wolf"