Karn, Silver Golem is a rare Legendary Artifact Creature — Golem card from Urza's Saga (USG), collector number 298. Current market price: $21.14, TCGplayer market prices via Scryfall.
Whenever Karn blocks or becomes blocked, it gets -4/+4 until end of turn. {1}: Target noncreature artifact becomes an artifact creature with power and toughness each equal to its mana value until end of turn.