Delenar was an ancient elven temple dedicated to the Seldarine that predated the Calamity. Once a holy place, it became twisted and dark with the arrival of Ourys Demethar, a drow that betrayed his own people and turned the facility into a laboratory of disease with the assistance of the influential Caerlinn family.
When the drow discovered that Ourys had betrayed them, they besieged the temple, but were unable to enter the facility due to the protection of a minor mythal that Ourys had manipulated to keep his former kin at bay. In the years that followed, it became clear that the drow was not about to give up on their siege, and Ourys decided to twist the magic of the mythal to keep everyone alive in death, causing the inhabitants to become undead in the process.
When a party of explorers arrived from Stormhaven, the inhabitants were dealt with, and Ourys was finally slain. Following the cleansing of the area, the former temple grounds were annihilated by a powerful yochlol in the service of Lolth, the Spider Queen.