Karn, Legacy Reforged 
Legendary Artifact Creature — Golem
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.
Legality
standardNOT LEGALpioneerLEGALmodernLEGALlegacyLEGALvintageLEGALcommanderLEGAL