Karn, Legacy Reforged is a mythic Legendary Artifact Creature — Golem card from March of the Machine: The Aftermath (MAT), collector number 99. Current market price: $20.16 (foil $19.24), 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.