Act 5 Quest 4: Betrayal of Harrogath - Diablo 2
Quest Location: Halls of Vaught/Nihlathak's Temple
Quest NPC: Anya
Quest Goal: Defeat Nihlathak
Reward: Personalize an item
Flag: Portal to Pindleskin
This quest begins when you speak with Anya after freeing her from her icy prison in the Frozen River during Act V Quest 3: Prison of Ice. She'll open a portal to Nihlathak's temple right next to her in the town of Harrogath which will give you extremely easy access to a Super Unique by the name of Pindleskin.
Pindleskin is one of the most well known farms in Diablo 2 largely due to how easy he is to access from town through this very portal. If you plan to farm Pindleskin it's very important you remember not to grab the Waypoint in the Halls of Pain as this will close the portal. You can complete this quest and defeat Nihlathak so long as you don't get the Waypoint this portal will remain open. For more information about farming Pindleskin follow the link.
Above is a screenshot of the final floor of the dungeon with Nihlathak called the Halls of Vaught. The layout of this floor is always the same in every game/difficulty, you always enter in the central area and you can pick from any of the four directions. The only thing that actually changes is what side Nihlathak spawns on.
In the screenshot above if you look in the upper right hand corner you'll see a platform on the map, this is the icon that marks Nihlathak's location on the map. Nihlathak's platform is always swarmed with dozens of minions that are pretty easy to dispatch. Like the Necromancer he is, Nihlathak likes to cast Corpse Explosion on his fallen comrades as well as use Curses to weaken you and your party.
Nihlathak is a popular farm on Hell difficulty since he drops the same items as Baal but takes 1/4th the time to actually defeat. Be warned, however, that the enemies on Nihlathak's floor and Nihlathak himself are no joke on Hell difficulty. For more information about farming Nihlathak follow the link provided.