Mavos Siloreth

 is an ancient Chimer mage, who played an important role during the Nede-Chimer War.

The War and Balreth
Mavos was a sorcerer during the Nede-Chimer War of the First Era and, according to him, the most  powerful mage of his time.

During the decisive battle where General Balreth and his brother sacrificed themselves, creating the creatures known as Brothers of Strife, Mavos was fighting for Morrowind against the Nede invaders.

The Chimer won the Nedes on that battle, but with a great cost. Balreth started killing the Chimer forces, and after a great lost of lifes, Mavos imprisoned Balreth with two summoned Fire Atronachs, Blaze and Ragebinder.

Interactions
In the course of The Elder Scrolls Online, the Soulless One requests Mavos' help to capture Balreth again. Mavos, as a ghost, guides the Soulless One in obtaining the necessary ingredients, so the Soulless One could have a vision of the past and see how Balreth was imprisoned.

The Soulless One goes to Ash Mountain, where Balreth is obtaining strenght. Soulless One makes the rituals and the two guardian Atronachs, Blaze and Ragebinder, are summoned. Then, the two atronach open a portal to the center of the Mountain.

Mavos appears one more time and speaks with the Soulless One. After Balreth is defeated, Mavos is not seen again.

Trivia

 * While Mavos says that he is the most powerful mage of his time, there are no doubts that Shalidor is a superior. However, Mavos may be talking about the years which he lived, not the whole First Era.