Activate Grim Poppet by removing a -1/-1 counter from it, targeting another creature.
Repeat until Grim Poppet has no -1/-1 counters on it.
Activate Antler Skulkin by paying {2}, giving Grim Poppet persist until end of turn.
Activate Ashnod's Altar by sacrificing Grim Poppet, adding {C}{C}.
Grim Poppet dies, triggering its persist ability, returning it from your graveyard to the battlefield with a total of four -1/-1 counters on it.
Repeat.

Infinite -1/-1 counters

Infinite death triggers

Infinite ETB

Infinite LTB

Infinite sacrifice triggers

Elesh Norn, Grand Cenobite

Grim Poppet

Painter's Servant

Antler Skulkin

Ashnod's Altar

Activate Grim Poppet by removing a -1/-1 counter from it, targeting another creature.
Repeat until Grim Poppet has no -1/-1 counters on it.
Activate Antler Skulkin by paying {2}, giving Grim Poppet persist until end of turn.
Activate Krark-Clan Ironworks by sacrificing Grim Poppet, adding {C}{C}.
Grim Poppet dies, triggering its persist ability, returning it from your graveyard to the battlefield with a total of four -1/-1 counters on it.
Repeat.

Krark-Clan Ironworks

Crovax, Ascendant Hero

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