A circle of sugar and a word of forbiddance.

Oblivion Ring

When this enchantment enters, exile another target nonland permanent.
When this enchantment leaves the battlefield, return the exiled card to the battlefield under its owner's control.

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Modern Masters 2015

When Oblivion Ring enters the battlefield, exile another target nonland permanent.
When Oblivion Ring leaves the battlefield, return the exiled card to the battlefield under its owner's control.

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If Oblivion Ring leaves the battlefield before its first ability has resolved, its second ability will trigger and do nothing. Then its first ability will resolve and exile the targeted nonland permanent forever.

If there are no nonland permanents on the battlefield other than an Oblivion Ring, and the card it exiled was another Oblivion Ring, casting a third Oblivion Ring will result in an involuntary infinite loop that will end the game in a draw (unless someone chooses to break it by putting another nonland permanent onto the battlefield or destroying one of the Oblivion Rings, for example).

If the exiled card is an Aura, that card’s owner chooses what it will enchant as it comes back onto the battlefield. An Aura put onto the battlefield this way doesn’t target anything, but the Aura’s enchant ability restricts what it can be attached to. If the Aura can’t legally be attached to anything, it remains exiled forever.

Auras attached to the exiled permanent will be put into their owners’ graveyards. Equipment attached to the exiled permanent will become unattached and remain on the battlefield. Any counters on the exiled permanent will cease to exist.