Call to Glory in exile (exiled by Isochron Scepter).
Isochron Scepter and The Blackstaff of Waterdeep on the battlefield.
Creatures you control can tap to produce at least ({3} magic symbol) .
({U} magic symbol) available.
Steps
Activate all mana-producing creatures you control by tapping them, adding at least ({3} magic symbol) .
Activate The Blackstaff of Waterdeep by paying ({1} magic symbol) ({U} magic symbol) and tapping it, causing Isochron Scepter to become a 4/4 artifact creature.
Activate Isochron Scepter by paying ({2} magic symbol) and tapping it, casting a copy of Call to Glory without paying its mana cost.
Resolve Call to Glory, untapping all creatures you control.
Activate all mana-producing creatures you control by tapping them, adding at least ({3} magic symbol) .