The Soul Stone
Legendary Artifact — Infinity Stone
Oracle Text
Indestructible
: Add 
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, , Exile a creature you control: Harness The Soul Stone. (Once harnessed, its ∞ ability is active.)
∞ — At the beginning of your upkeep, return target creature card from your graveyard to the battlefield.
Rules for this card
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A keyword ability that precludes a permanent from being destroyed. See rule 702.12.
Rulings (3)
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Wizards of the CoastBeing harnessed isn't copiable. If something else becomes a copy of The Soul Stone, it must be harnessed separately. Similarly, if The Soul Stone is already harnessed and becomes a copy of something else, it stays harnessed, though it's very likely that won't be relevant until that copy effect ends.
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Wizards of the CoastUntil it is harnessed, The Soul Stone doesn't have the ability listed after the infinity symbol. It also doesn't have that ability in zones other than the battlefield.
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Wizards of the Coast"Harnessed" is a designation that the permanent has once an ability instructs you to harness it. It has no special rules meaning other than being a designation that the ∞ ability (and, theoretically, other effects) can see.