Criatura artefacto legendaria — Gólem
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Tanto la fuerza como la resistencia de Karn, legado reforjado son iguales al mayor valor de maná entre los artefactos que controlas.
Al comienzo de tu mantenimiento, agrega

por cada artefacto que controlas. Este maná no se puede usar para lanzar hechizos que no sean de artefacto. Hasta el final del turno, no pierdes ese maná en cuanto terminan los pasos y las fases.
The mana produced by Karn's last ability can be spent on anything that isn't a nonartifact spell. This includes casting artifact spells, paying costs to activate abilities of both artifact and nonartifact permanents, paying ward costs, and so on.
Karn's last ability isn't a mana ability, even though it adds mana. It uses the stack and it can be responded to. Use the number of artifacts you control as the ability resolves to determine how much mana to add.
The ability that defines Karn's power and toughness works in all zones, not just the battlefield. As long as Karn is under your control and still an artifact, its own mana value will count. In most cases, it'll be at least 5/5.