Declare the Combat Celebrant and Savior of Ollenbock as an attacker, choosing to exert Combat Celebrant as it attacks.
Combat Celebrant and Savior of Ollenbock triggers.
Resolve the Combat Celebrant triggers, untapping all other creatures you control and causing you to get an additional combat phase after this one.
Resolve the Savior of Ollenbock triggers, holding priority, activate Ashnod's Altar by sacrificing Combat Celebrant, adding ({C} magic symbol) ({C} magic symbol) .
Resolve the Savior of Ollenbock triggers, putting a +1/+! counter on itself and exiling Combat Celebrant from your graveyard.
Activate Ashnod's Altar by sacrificing Savior of Ollenbock, adding ({C} magic symbol) ({C} magic symbol) .
Savior of Ollenbock dies, triggering Nim Deathmantle and Savior of Ollenbock.
Resolve the Nim Deathmantle trigger, causing you to pay ({4} magic symbol) to return Savior of Ollenbock from your graveyard to the battlefield, and attach Nim Deathmantle to it.
Resolve the Savior of Ollenbock trigger, putting Combat Celebrant onto the battlefield.