Declare Combat Celebrant and Unforgiving One as attackers, choosing to exert Combat Celebrant as it attacks.
Combat Celebrant triggers, untapping all other creatures you control and causing you to get an additional combat phase after this one.
Unforgiving One triggers, returning Mirror Image from your graveyard to the battlefield as a copy of Combat Celebrant.
Activate Altar of Dementia by sacrificing Combat Celebrant, causing target player to mill cards equal to Combat Celebrant's power.
Move to your next combat phase.
Declare Mirror Image and Unforgiving One as attackers, choosing to exert Mirror Image as it attacks.
Mirror Image triggers, untapping all other creatures you control and causing you to get an additional combat phase after this one.
Unforgiving One triggers, returning Combat Celebrant from your graveyard to the battlefield.
Activate Altar of Dementia by sacrificing Mirror Image, causing target player to mill cards equal to Mirror Image's power.
Repeat.

Infinite combat phases

Infinite mana creatures you control can produce

Infinite mill

Infinite self-mill

Infinite untap of creatures you control

Combat Celebrant

Unforgiving One

Mirror Image

Altar of Dementia

Mass Hysteria

Anger

Declare Combat Celebrant and Unforgiving One as attackers, choosing to exert Combat Celebrant as it attacks.
Combat Celebrant triggers, untapping all other creatures you control and causing you to get an additional combat phase after this one.
Unforgiving One triggers, returning Clone from your graveyard to the battlefield as a copy of Combat Celebrant.
Activate Altar of Dementia by sacrificing Combat Celebrant, causing target player to mill cards equal to Combat Celebrant's power.
Move to your next combat phase.
Declare Clone and Unforgiving One as attackers, choosing to exert Clone as it attacks.
Clone triggers, untapping all other creatures you control and causing you to get an additional combat phase after this one.
Unforgiving One triggers, returning Combat Celebrant from your graveyard to the battlefield.
Activate Altar of Dementia by sacrificing Clone, causing target player to mill cards equal to Clone's power.
Repeat.

Clone

Combos with "Unforgiving One"