Karn, Legacy Reforged (MAT-049) - March of the Machine: The Aftermath

Karn's power and toughness are each equal to the greatest mana value among artifacts you control.
At the beginning of your upkeep, add 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.