Pulsemage Advocate does not have summoning sickness.
You control a nontoken creature with power three or greater.
Steps
Activate Altar of Dementia by sacrificing a creature with power three or greater.
The creature dies, triggering Pulsemage Advocate, untapping it.
Resolve the Altar of Dementia trigger, causing target opponent to mill cards equal to the sacrificed creature's power.
Activate Pulsemage Advocate by tapping it and putting three cards from an opponent's graveyard into their owner's hand, returning the creature from your graveyard to the battlefield.