Mirrex is a rare Land — Sphere card from Phyrexia: All Will Be One Promos (PONE), collector number 254p. Current market price: $1.02 (foil $0.94), 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.)