Mirrex is a rare Land — Sphere card from Phyrexia: All Will Be One (ONE), collector number 400. Current market price: $1.05 (foil $1.39), TCGplayer market prices via Scryfall.
{T}: Add {C}. {T}: Add one mana of any color. Activate only if this land entered this turn. {3}, {T}: Create a 1/1 colorless Phyrexian Mite artifact creature token with toxic 1 and "This token can't block." (Players dealt combat damage by it also get a poison counter.)