Koilos was a mountainous location deep in the Western Desert on Terisiare (Dominaria). It lay between the Sardian Mountains and the Kher Ridges. At the time of the Brothers' War, a few crumbling buildings and a network of caves containing the last technological remnants of the Thran were all that remained of Halcyon, the beautiful capital of the Thran Empire. Urza named the place "Koilos", the High Argivian word for "secret".
The most important thing here was a portal in the Caves of Koilos that would lead to Phyrexia. During the War, it was the headquarters of Gix, a powerful Phyrexian demon, who manipulated both sides in the war for Phyrexia's best interests.
After the Ice Age the site became accessible again. Once more the portal opened and Yawgmoth came through to invade Dominaria, but the Weatherlight sealed it again. Currently, Koilos is located in New Argive.
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