Klauth, Unrivaled Ancient
Legendary Creature - Dragon | Power/Toughness: 4 / 4 (CMC 7)
Whenever Klauth, Unrivaled Ancient attacks, add X mana in any combination of colors, where X is the total power of attacking creatures. Spend this mana only to cast spells. Until end of turn, you don't lose this mana as steps and phases end.