The Mightstone and Weakstone is a rare Legendary Artifact — Powerstone card from The Brothers' War (BRO), collector number 238. Current market price: $4.26 (foil $5.55), 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.)