Modular Monstrosity is a rare Artifact Creature — Construct card from Unstable (UST), collector number 155. Current market price: $0.19 (foil $2.60), TCGplayer market prices via Scryfall.
Whenever an opponent casts a spell, you have five seconds to choose a keyword you haven't chosen for a card named Modular Monstrosity today that's been printed on a creature card. If you do, this creature gains that ability. Otherwise, this creature loses all keyword abilities.