The Undying Temple is a tower within Doth'Mordun that was central to the drow plot to escape their prison. Wrapped around a fragment of the Negative Energy Plane obtained by Kiaransalee, the drow used it to power several portals out of their prison.
However, the Brightstone Heroes entered through one of these portals, ascended the tower, and destroyed the fragment with a massive infusion of positive energy, shutting down the portals. In the years that followed, the Negative Energy Plane leaked through the gap created by the fragment's destruction into the Plane of Shadow, creating the Shadowfell and dragging the entire valley of Doth'Mordun into it. It is unknown what happened to the Undying Temple after the Negative Energy Plane fragment's destruction.