Klauth, Unrivaled Ancient

Flying, haste
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.

The List

Forgotten Realms Commander

Despite that fact that it can produce mana as it resolves, Klauth's triggered ability is not a mana ability because its trigger condition wasn't the activation or resolution of an activated mana ability. This means that it uses the stack and can be responded to. Notably, players can respond by destroying attacking creatures to reduce the amount of mana produced.