Test Of The Twins by Margaret Weis

An intense conclusion to a saga in which the fraught bond between twin brothers—one a loyal warrior, the other a brilliant, dangerously ambitious mage—comes under ultimate strain as the mage pursues godlike power. Time-bending sorcery, the legacy of a long-dead archmage, and a small circle of allies draw them into desperate encounters across past and present; loyalties are tested, painful sacrifices are made, and the true cost of obsession and the limits of love and duty are revealed.

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