Karn, Silver Golem is a rare Legendary Artifact Creature — Golem card from World Championship Decks 1999 (WC99), collector number kb298. Current market price: $7.62, 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.