In order to obtain a Hellfire Ring or Amulet, one has to work their way through the Uber Boss system of harvesting machines and organs to produce a legendary ring or amulet (official blog post).
To create the infernal machine required to open the portals to the Ubers, the keys are drops from mobs nick named Key Wardens.
Where are the various key wardens? Are there any requirements for making the machines drop? What’s the drop rate on different difficulty settings?
This post has been updated for 2.0.1 to only contain the locations of the keywardens.
The updated drop rates can be found in this answer:
Key of Destruction
- Odeg the Keywarden is in Fields of Misery in act 1. Odeg always spawns with the molten effect, and he has a big fireball spell that can do ok damage. He also has the jail affix and likes to kite a lot, making it difficult to engage him as a melee character.
Key of Hate
Sokahr the Keywarden spawns in Dahlgur Oasis. Dahlgur oasis is quite big and he can spawn anywhere in that zone, so having multiple people helps a lot. Sokahr has an electric attack and a whirlwind attack. When he starts whirlwinding, I recommend to back up since that attack hurts a lot. He also has mortars.
Another note here is that he is a cake walk for a demon hunter using the ball lightning attack. Since he often (always?) appears with a slow missile shield these balls hang around sparking on him if you can stay far enough away that he isn’t chasing you. Alternately make him chase you in a circle so the balls stay within his shield.
Key of Terror
- Xah’Rith the Keywarden is in Stonefort. Xah’Rith is the hardest of the 3 key wardens as he teleports around a lot and have a high damage aoe attack. If you are a melee character, it helps a lot if you lure him to one of the many stair cases in the area so he can’t run away as easily.
Infernal Machine Blacksmithing Plans
- Dropped from Nekarat the Keywarden in Act 4. Has very high damage and aoe. He also has a chance to drop any of the 3 keys.