Archon of Falling Stars and Altar of Dementia on the battlefield.
Steps
Activate Altar of Dementia by sacrificing Archon of Falling Stars.
When Archon of Falling Stars dies, it triggers, returning Dance of the Dead from your graveyard to the battlefield.
When Dance of the Dead enters the battlefield, it triggers, returning Archon of Falling Stars from your graveyard to the battlefield, and attaching Dance of the Dead to it.
Resolve the Altar of Dementia ability, causing target player to mill cards equal to Archon of Falling Stars' power.