Pros:
- Lots of AoE Packs
- Most of the time 1 Champion (Blue)
- Sometimes 1 Rare (Yellow)
- Quick EXP
- Great way to stock up on potions, gems, and tomes.
- Sometimes there are treasure goblins at the entrance.
Cons:
- Depending on gear, it can be rough.
- Each time you enter the lay out is different so are the monsters.
- Might take at least 5 minutes per run, depending on your rares/champions
- Invulnerable Monsters on Hell
- Annoying Companions
Act 2 - Chapter 7 Part 3 "Zoltun Kulle's Blood"
Preferred Difficulty: Hell
The area is Vault of the Assassin.
I did this on Hell difficulty with pretty good gear. YMMV.
When you enter head up. Sometimes there will be enemies, usually a little or densely populated. Sometimes there will be a yellow/blue mob.
Once you clear those there's usually a cross road. One that leads 3 directions or one that leads to two leading left and right. Go to each direction to the next area of the room, try to find the area with more monsters in it. After the next room, if you feel that there's more monsters keep going. But don't go past 2 areas unless there's more.
Back track and go the other direction and do exactly what you did.
After each side is cleared, teleport, sell, repair, and leave game. When you enter again you should start back at the Vault's entrance.
Example:
Since this is a natural farm spot, Blizzard most likely won't nerf it. If they do they will probably reduce Champion spawns/Monster density.
Let me know how it goes for you, and which difficulty.
[Diablo III] Vault of the Assassin Farming - YouTube
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I found another farming spot, but it's much smaller but takes 1-2 minutes. After you get both blood, enter the Terminus area and go to the Storm Halls. At the very end unlock the area. But, as you can see, there's 3 different paths. One leading to the portal out and 2 where monsters are. Each side can contain a champion or rare pack.
Clear both sides, town, exit game, rejoin and you'll be back at the last area. Check all sides for rares/champions.