You control another Illusion with power five or greater.
Steps
Activate Altar of Dementia by sacrificing another Illusion with power five or greater.
The Illusion dies, triggering Minn's second ability, putting Fool's Demise from your hand onto the battlefield attached to Phantasmal Dreadmaw.
Resolve the Altar of Dementia ability, causing target player to mill cards equal to the sacrificed creature's power.
Activate Altar of Dementia by sacrificing Phantasmal Dreadmaw.
Phantasmal Dreadmaw dies, triggering Minn's second ability and Fool's Demise's first and second abilities.
Resolve the second Fool's Demise trigger, returning it from your graveyard to your hand.
Resolve the first Fool's Demise trigger, returning Phantasmal Dreadmaw from your graveyard to the battlefield.
Resolve the Minn trigger, putting Fool's Demise from your hand onto the battlefield attached to Phantasmal Dreadmaw.
Resolve the Altar of Dementia ability from step 4, Resolve the Altar of Dementia ability, causing target player to mill cards equal to Phantasmal Dreadmaw's power.