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.

Flash You may cast colorless spells as though they had flash. Whenever you cast a colorless spell, until end of turn, this creature gains trample and gets +X/+X, where X is that spell's mana value.