All permanents on the battlefield (In Bolas's Clutches attached to Boonweaver Giant).
Steps
Activate Phyrexian Altar by sacrificing Boonweaver Giant, adding one mana of any color.
When Boonweaver Giant dies as a legendary creature due to In Bolas's Clutches, 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 In Bolas's Clutches from your graveyard to the battlefield attached to the token.