Sentry Bot is a rare Artifact Creature — Robot card from Fallout (PIP), collector number 371. Current market price: $0.49 (foil $0.60), TCGplayer market prices via Scryfall.
Flash This spell costs {1} less to cast for each creature attacking you. When this creature enters, you get {E} for each creature attacking you. At the beginning of combat on your turn, you may pay {E}{E}{E}. If you do, put a +1/+1 counter on each creature you control.