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
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
Infernal Machine Blacksmithing Plans
The Reaper of Souls update has substantially modified this process from the launch version.
There are 4 different machines that can be created which correspond to the particular fights to obtain a particular organ. No more opening 3 portals to guarantee seeing the uber you need.
The machines spawn directly from the Keywardens. As of patch 2.3 each member of a party has an independent probability of generating a machine.
The Infernal Machine plan can drop from any of the Keywardens and they all have a small chance to spawn a second key that is not particular to them.
The level 70 Hellfire Ring requires one of all four organs, the new crafting plan which costs 5,000,000 gold coins from Squirt in Act 2 and 50,000 coins to craft.
The organs are:
It is still possible to obtain the original keys while you are level 60 and to craft the level 60 Hellfire Ring as per the legacy information that used to be contained in answers here (you can see that info from reviewing edits on the top answers).
It is not possible to stack the level 60 and 70 Hellfire Rings to obtain an 80% experience gain bonus.
The key rates are reported to be but are not universally accepted as:
The organ drop rates are estimated to be:
Lylirra made a blue post about drop chances. You have to have 5 stacks of Nephalim Valor and the drop chance scales up to 100% with monster power level.
You'll need five stacks of NV in order for the keys and the demonic organ pieces to drop.
Currently, both the keys and the demonic organ pieces (Vengeful Eye, Devil's Fang, and Writhing Spine) have higher chances to drop in higher MP levels, all the way up to 100% in MP10.
When they appear, they'll show us as legendary items, making them easy to distinguish from the other items the mobs may pop.
Diablofans posted information on the location of all of the keys:
- Key 1 - Found on Goat Mutant Key Warden in the Fields of Misery (Act 1)
- Key 2 - Found on Dune Dervish Key Warden in Dahlgur Oasis (Act 2)
- Key 3 - Found on Morlu Spellcaster Key Warden in Stonefort (Act 3)
- Infernal Machine Plan - Found on Terror Demon Key Warden in Silver Spire Lvl 1 (Act 4)
- The Key Wardens will always spawn in each Inferno game.
As verified by a Blizzard employee, this is the up-to-date information about keys, in Patch 2.0.1:
1) You don't need to do anything special; just kill the Key Wardens
2) Keys only drop on Torment difficulty
3) Key drop rates:
Since 2.0.1 and the remove of NV buff. Keys and others stuffs can drop at anytime.
