Lord Kasilas Evengard
You know him only as an all-powerful, ethereal voice.
In his life, Kasilas served Vecna in his quest to ascend to deityhood. That is, until Kasilas decided that he could not fulfill Vecna’s evil wishes. In a climactic battle, two of Vecna’s lieutenants were lost, and the third (Toras) was stripped of much of his power. Unfortunately, Vecna was still able to rise to become a deity, and sealed Kasilas in a terrible prison.
Somehow, your party was able to find Kasilas’ prison and return a collection of writings to him, which Kasilas then revealed to be a book that would connect your world to the greater world at large. Once this was done, Kasilas released his hold on life, destroying both him and his ethereal prison.