Threshold — Sara Douglass

Over the hot southern land of Ashdod looms the V shadow of Threshold, the pyramid that the Magi of Ashdod are building to propel themselves into Infinity. Over the years, thousands of slaves have given their lives to the construction of Threshold. Now construction is almost complete; the Magi need only to add the finishing touches.

The Master of the Magi knows the glassworker slave Tirzah is hiding something, but he would never guess her secret is forbidden magic. Tirzah can communicate with glass--and the glass in Threshold screams to her in pain. For it knows what neither Tirzah nor any of the Magi suspect: Something waits in Infinity, watching, biding its time, and when the final glass plate is laid and the capstone cemented in blood, it plans to use Threshold to step from Infinity into Ashdod....