The Mightstone and Weakstone is a rare Legendary Artifact — Powerstone card from The Brothers' War Promos (PBRO), collector number 238s. Current market price: — (foil $12.30), TCGplayer market prices via Scryfall.
When The Mightstone and Weakstone enters, choose one — • Draw two cards. • Target creature gets -5/-5 until end of turn. {T}: Add {C}{C}. This mana can't be spent to cast nonartifact spells. (Melds with Urza, Lord Protector.)