All permanents on the battlefield (On Serra's Wings attached to Boonweaver Giant).
Steps
Activate Altar of Dementia by sacrificing Boonweaver Giant.
When Boonweaver Giant dies as a legendary creature due to On Serra's Wings, Ratadrabik of Urborg triggers, creating a token copy of Boonweaver Giant that is a 2/2 Zombie in addition to its other types.
When the Boonweaver Giant token enters the battlefield, it triggers, returning On Serra's Wings from your graveyard to the battlefield attached to the token.
Resolve the Altar of Dementia ability, causing target player to mill cards equal to Boonweaver Giant's power.