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The Conqueror's Brigade first discovered the existence of portal stones at the conclusion of the Dark Tower Adventure when Iscus transported Dur-Adn, Honorable John, Gorlain, and Avrik to the Tower of Ulission in Urdistau, The Dead Lands. There, embedded into a pentacle upon the door were several stones (King of Swords, Episode 1, recording 3, minute 59). The twenty-one white gems were laid out in a pattern: ten form the star at its endpoints and intersections, plus one in the middle, and ten more form the circle of the pentacle.
During his audience with Kalanan a the start of the Bow and Bone adventure, Gorlain was given one of the portal stones and instructed to find the rest…
Later, during episodes of the Tower of Ulission, Dur-Adn the portal-mage has learned that the Dead Lands are littered with portals, and indeed the entire spindle world, although they are most concentrated in the Dead Lands. And each has a key, typically a gem, although he has not yet discerned what if any relationship exists between the tower stones and the portals…