Walking Ballista is a rare Artifact Creature — Construct card from Mystery Booster 2 (MB2), collector number 238. Current market price: $8.88 (foil $46.57), TCGplayer market prices via Scryfall.
This creature enters with X +1/+1 counters on it. {4}: Put a +1/+1 counter on this creature. Remove a +1/+1 counter from this creature: It deals 1 damage to any target.