The Seventh Gate ~ Margaret Weis; Tracy Hickman
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But a terrible decision awaits them: To save Haplo they must enter the forbidden Seventh Gate. If they do so, they risk betraying its location to Lord Xar, who will use it to claim his terrible dominion over the worlds. Suddenly all their lives--and the fate of the sundered realms--hang in the balance. The final chapter is about to unfold in the apocalyptic struggle between good and evil....