The Summoner

It concerns two Mage brothers, who plan to terrorize the city of Mournhold with their powers.

Background
Gavis Velas is a feared summoner in Mournhold. His less talented brother Ovis Velas strives to take the credit for Gavis' actions and eventually attacks the Nerevarine on the Great Bazaar, assuming they would be a easy fight. Killing him starts the quest.

Quick walkthrough

 * Kill Ovis Velas and take the key to Velas Manor from his body.
 * Talk to Drathas Reyas.
 * Head to Velas Manor.
 * Kill Gavis Velas.