Karn, Legacy Reforged is a mythic Legendary Artifact Creature — Golem card from March of the Machine: The Aftermath (MAT), collector number 184. Current market price: $19.77 (foil $21.81), TCGplayer market prices via Scryfall.
Karn's power and toughness are each equal to the greatest mana value among artifacts you control. At the beginning of your upkeep, add {C} for each artifact you control. This mana can't be spent to cast nonartifact spells. Until end of turn, you don't lose this mana as steps and phases end.