Sotha Sil is a member of the Tribunal found within his dome in Clockwork City. Following the release of his creatures on Mournhold, Almalexia believes that Sotha Sil has gone mad after losing his godhood, and sends you to kill him.
In 3E 417, Sotha Sil, along with his fellow Tribunal Almalexia and Vivec, traveled to Red Mountain to enact their annual godhood ritual on the Heart of Lorkhan. However, Dagoth Ur and his Ash Vampires attacked the Tribunal, stealing Keening and Sunder from them.
Sotha Sil later retired to his Clockwork City, and cut off what little interaction with the outside world he had left. When the Nerevarine destroyed the heart in 3E 427, Vivec noted that he doubted Sotha Sil even noticed the loss of his godhood due to his obsession with his city.
Later, Almalexia sent the Nerevarine to Clockwork City to kill Sotha Sil. However, Sotha Sil was already found dead, and Almalexia traveled to them revealing her true plan to have the Nerevarine killed by Clockwork City's defenses on their way to Sil, and that she killed Sotha Sil out of her insanity.
Following the release of the Fabricants and other creatures on Mournhold, Almalexia sends you to kill Sotha Sil in Clockwork City, out of the belief he is behind the attack. After defeating his Imperfect bodyguards, Sotha Sil can be found his dome. However, after interacting with him, the following message appears:
After walking back to the entrance, Almalexia will appear, informing you of her true plans. She will say that she wanted the Nerevarine to become a martyr in her name, who died trying to defeat the mad god Sotha Sil. She reveals that she was the one who killed Sotha Sil, and needs to kill you in order to keep her dark plan a secret.