Convoke the Unassailable

Convoke (Your creatures can help you cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
Buyback {3} (You may pay an additional {3} as you cast this spell. If you do, put this card into your hand as it resolves.)
Create a white 7/7 Legendary Creature token named Bruthos, the Unassailable with flying and indestructible.