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.
Activate
Ashnod's Altar by sacrificing the nontoken creature with a coin counter on it, adding ({C} magic symbol) ({C} magic symbol) .
When the creature dies, it 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 target creature you control.