Act 2 Quest 3: Tainted Sun - Diablo 2
Quest Location: Lost City/Claw Viper Temple
Quest NPC: Drognan
Quest Goal: Destroy the altar in Claw Viper Temple
Reward: Darkness lifted
Flag: The permanent darkness will be gone
The Tainted Sun quest begins the moment you step into the Lost City during Act 2 and causes it to become dark throughout all of Act 2 until it's completed. The first step of this quest will be to speak to Drognan about the strange darkness but this step isn't actually required to complete the quest.
For this quest you'll need to travel to the Claw Viper Temple which you can reach by first going to the Lost City then to Valley of the Snakes before finally arriving at the Claw Viper Temple. You'll find our quest objective on the second level of the temple, it's a small altar guarded by Fangskin who is a lightning enchanted Super Unique enemy.
In addition to this quest being completed here, you'll also be able to pick up the Claw Viper Amulet from the destroyed altar which is used in Act 2 Quest 2: The Horadric Staff.
Tainted Sun Altar in Claw Viper Temple.