Kill the shadow dragon
Thaxll'ssillyia in the ruined temple of
Amaunator. You can get there during the quest
Investigate the Deaths in the Umar Hills.
*** Spoiler - click to reveal ***The easiest way to kill Thaxll'ssillyia is to transform one of your characters into Mustard Jelly either by using the Cloak of Sewers or Polymorph Self spell. Mustard Jelly is practically immune to the dragon's attacks.