Hedron prison

The lithomancer constructed a massive network of stone diamonds called hedrons, whose power would form the bars of a planewide prison, forever preventing the Eldrazi from leaving.

The planeswalkers concentrated the power of their imprisonment spell in a secret location deep inside Zendikar’s mountains of Akoum: a subterranean chamber called the Eye of Ugin. To make sure that the imprisonment would not be broken, the three planeswalkers sealed the chamber with a kind of mystical lock: the Eye of Ugin could only be reactivated by the presence of three planeswalker sparks—and the colorless, invisible breath of Ugin, the spirit dragon himself.

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THE ELDRAZI ARISEN