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

Infinite death triggers

Infinite creature ETB

Infinite creature LTB

Agatha's Soul Cauldron

Walking Ballista

Nearheath Stalker

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

Triskelion

Activate Altar of Dementia by sacrificing Nearheath Stalker
When Nearheath Stalker dies, its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the Altar of Dementia ability.
Activate Altar of Dementia by sacrificing Nearheath Stalker
When Nearheath Stalker dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Nearheath Stalker enters and dies as a state-based action due to having 0 toughness. Its undying ability triggers, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the Altar of Dementia ability from step 4.
Repeat from step 4.

Infinite mill

Infinite creature sacrifice triggers

Infinite self-mill

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

Altar of Dementia

Activate Champion of the Weird by paying 1 life and putting two -1/-1 counters on Nearheath Stalker.
Nearheath Stalker dies due to having 0 toughness, triggering its undying ability and Dross Harvester
Resolve the Dross Harvester trigger, causing you to gain 2 life.
Resolve the Nearheath Stalker trigger, returning it from your graveyard to the battlefield with a +1/+1 counter on it.
Resolve the Champion of the Weird ability from step 1, causing an opponent to blight 2.
Activate Champion of the Weird by paying 1 life and putting two -1/-1 counters on Nearheath Stalker, causing an opponent to blight 2.
Repeat.

Infinite -1/-1 counters

Infinite lifegain triggers

Champion of the Weird

Dross Harvester

Combos with "Nearheath Stalker"