Athreos, Shroud-Veiled + Grimgrin, Corpse-Born + Reluctant Role Model Combo

All permanents on the battlefield.

You control at least one additional nontoken creature with a coin counter on it.

Activate Grimgrin by sacrificing the creature with a coin counter on it.

When the creature dies, Athreos's second ability and Reluctant Role Model's ability trigger.

Resolve the Reluctant Role Model trigger, putting a coin counter onto itself as well as +1/+1 counters if those were present.

Resolve the Athreos trigger, returning the creature from your graveyard to the battlefield.

Resolve the Grimgrin ability from step 1.

Activate Grimgrin by sacrificing Reluctant Role Model.

When Reluctant Role Model dies, it and Athreos's second ability trigger.

Resolve the Reluctant Role Model trigger, putting a coin counter and +1/+1 counters onto the other nontoken creature.

Resolve the Athreos trigger, returning Reluctant Role Model from your graveyard to the battlefield.

Resolve the Grimgrin ability from step 6.

Repeat.

Infinite +1/+1 counters on a creature

Infinite creature ETB

Infinite creature LTB

Infinite creature sacrifice triggers

Infinite death triggers