Diablo II/Tainted Sun

The third quest is activated when you enter the Lost City, beyond the, for the first time, once the sky darkens. It's necessary to complete as doing so also allows you to get one step closer to the ../Horadric Staff/ required to complete the second and sixth quests, and with them the Act.

If you talk to anyone but Drognan upon first returning to town:

Talk to Drognan:

Lost City
The Lost City is randomly populated by three of the following five monster types: s, s, s, s and s. There can also be s.

The entrance to the Valley of Snakes is somewhere on the perimeter of the Lost City. If you explore, you can find a waypoint and a trap door down to the Ancient Tunnels, and you may encounter Dark Elder.

Ancient Tunnels
This is an optional area, randomly populated by three of the following four monster types:, electric s, s and s (so there's 75% chance of being populated exclusively by undead). There's also a chest guarded by a non-random monster, in addition to those randomly generated.

Valley of Snakes
The Valley of Snakes is a small area, not populated except for a possible or two. The entrance to the Claw Viper Temple is in the middle of it.

Claw Viper Temple Level 1
The first level is randomly populated by three of the following six monsters: s (2), s (2),, and s with s (this means Claw Vipers and Salamanders can be picked twice, so each has 80% chance of spawning, while Embalmed and Guardians each have 'only' 50% chance of spawning; there's also 80% chance of being populated exclusively by Claw Vipers and Salamanders). There can also be s and Mummy Sarcophagi, which can generate.

The most direct route to the second level is likely to be found by traveling left, in an anticlockwise direction. Before going down the stairs, you may want to cast a portal...

Claw Viper Temple Level 2
If you haven't asked Drognan about the strange darkness, you get a second prompt upon entering. Whether or not you ask him, if you return to town before completing this quest:

The second level is similarly populated to the first (except there can be no Fire Towers or Mummy Sarcophagi), but it's considerably smaller. The layout is always the same: you enter on the northeast side, and the Tainted Sun Altar is on a large earthen platform in the middle of the room to the southwest. The only way to walk up to this altar is via stairs cut in the southwest side of the platform.

Fangskin


However, Fangskin spawns in the south of the room, and due to its speed and that of its minions it's unlikely you can run up the stairs to the altar without being intercepted, even if other monsters don't obstruct you. It's best to hug the wall and try to clear the north half of the room first, to give you more room to maneuver in this confined space: a charging Claw Viper or Salamander always hits and cannot be blocked, so things can get ugly if you get swarmed.

If you can make it up the stairs or or  up there then it makes things considerably easier: Claw Vipers and Salamanders are too large to follow, so ranged attackers or casters can kill them in relative safety, while melee attackers can use the stairs at a much-needed choke point. Of course, once you're up there you don't need to kill anything if you can escape back up to the first level, or cast a portal.

Left click the Tainted Sun Altar to destroy it, restoring sunlight. Doing so also causes the, the headpiece for a ../Horadric Staff/, to drop out of it.

Deckard Cain only speaks the following if you talk to him first upon your return:

Don't bother talking to Jerhyn (again) until you've talked to Drognan about the ../Arcane Sanctuary/. Talking to Lysander and Meshif may remind you if you forgot to pick something up...

Talking to anyone completes this quest.