Klauth, Unrivaled Ancient is a mythic Legendary Creature — Dragon card from Secret Lair Drop (SLD), collector number 2499. Current market price: $27.95 (foil $24.34), TCGplayer market prices via Scryfall.
Flying, haste Whenever Klauth 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.