Enduring Bondwarden has at least one coin counter on it.
Steps
Activate
Shilgengar by sacrificing
Enduring Bondwarden.
When
Enduring Bondwarden dies, it and
Athreos' third ability trigger.
Resolve the
Enduring Bondwarden trigger, putting all counters from it onto another nontoken creature you control.
Resolve the
Athreos trigger, returning
Enduring Bondwarden from your graveyard to the battlefield.
When
Enduring Bondwarden enters the battlefield, its backup ability triggers, putting a +1/+1 counter on the creature who had a coin counter placed on it, and giving it the printed ability until end of turn.
Resolve the
Shilgengar ability from step 1.
Activate
Shilgengar by sacrificing the nontoken creature with a coin counter on it, adding one mana of any color.
When the creature dies, its ability from
Enduring Bondwarden and
Athreos' third ability trigger.
Resolve the creature's trigger, putting all counters from it onto
Enduring Bondwarden.
Resolve the
Athreos trigger, returning the creature from your graveyard to the battlefield.
Repeat.
Results
Infinite +1/+1 counters on a creature.
Infinite Blood tokens.
Infinite artifact tokens.
Infinite creature ETB.
Infinite creature LTB.
Infinite creature sacrifice triggers.
Infinite death triggers.
Metadata
Bracket tag
OddballBracket 2-3+
Probably 2 or 3, but hard to classify
1
2
3
4
5
1 · Exhibition5 · cEDH
Bracket tags are estimated from the cards and results of each combo, as a guideline rather than a strict classification.