On this page of the guide to Diablo 2 Resurrected you will learn how to start and complete the task of the Tained Sun.
The task Tainted Sun in Diablo 2 Resurrected can be started by simply entering the site The Lost City, connected by a passageway with Far Oasis. It could be worthwhile to delay starting the mission for as long as possible, and then take care of it immediately. While the quest is active, the entire game world is shrouded in darkness, making movement, exploration and combat much more difficult.
To continue the quest return to town and talk to Drognan. Head back to the Lost City and search for the Valley of Snakes entrance by moving along the rocks. In this small area you'll find the Claw Viper Temple entrance.
Slowly enter the chamber on the second floor, eliminating as many Salamanders as you can - later break through the skeletons to kill the healers. Open the tomb in the center to finish the quest. To truly complete it you need to talk to Drognan.
This area is important to the Horadric Staff quest - you can find the amulet here, which is a component of the staff.