You control at least one additional nontoken creature that does not have flying.
Steps
Activate Altar of Dementia by sacrificing the additional creature.
When the creature dies, Luminous Broodmoth triggers, returning the creature to the battlefield from your graveyard with zero flying counters due to Solemnity.
Resolve the Altar of Dementia ability from step 1, causing target player to mill cards equal to the sacrificed creature's power.