The Despana Treasury Key is needed to open the easternmost door to the treasury room in Ust Natha's Temple of Lolth that holds the Dragon Eggs.
It is provided by Phaere in the Female Fighters' Society after completing some tasks for her, as part of The Drow Summoning Ritual questline.
Description[]
| “ | This key, obtained from Phaere, will give you access to the Despana treasury room. | ” |