From Diablo Wiki
Keywardens are unique bosses who have a chance to drop the keys and plan required to activate the Infernal Machine and obtain a Hellfire Ring. Keywardens are only found on Inferno difficulty, and each one is randomly located in a given level, one per act.
To have a chance at one of the keys or the plan dropping, players need to kill the Keywardens with 5 stacks of Nephalem Valor. Higher levels of Monster Power increase the odds of a key dropping. Keywardens drop items like other Elite bosses, and can be a good source of legendaries and rares, but their special ability to drop keys is what makes them prized enemies.
All keys, the crafting plan, the Demonic Organs and the Hellfire Ring are BoA, which means they can not be traded for or even given away. Players must farm the ingredients themselves, though all players in a multiplayer game can use the portals to the various Uber battles.
- See the Infernal Machine article for full details.
Find the Keys and Plan
The first stage in unlocking the Infernal Machine is gathering the keys. These can be obtained only from four keywardens, each of which is located somewhere in a special area in each act. This entire event is Torment-only and only possible on Torment 1 or higher difficulty. Keywardens will not be seen on lower difficulty levels.
Players without the Infernal Machine plan (new accounts or new in a Season) will get the plan as a drop from the first Keywarden they kill at level 70.
- The Key of Bones may drop from Odeg the Keywarden in Act I - Fields of Misery.
- The Key of Gluttony may drop from Sokahr the Keywarden in Act II - Dahlgur Oasis.
- The Key of War may drop from Xah'Rith the Keywarden in Act III - Stonefort.
- The Key of Evil may drop from Nekarat the Keywardenin Act IV - Gardens of Hope 2nd Tier.
- Nekarat's location changed from Silver Spire Level 1 in Patch 2.2.
The keys, plans, organs, and machines are all Bind to Account so must be collected by each player individually. A character may have more than 1 of each key at a time, but each key is consumed by the crafting process, so players will need multiple copies of each key to build the multiple Infernal Machines required to explore each portal area and gather all the materials required to craft the Hellfire Ring or Hellfire Amulet.
In D3v2 and RoS, all Keywardens have one key they will always drop, plus a small random chance to drop another key. They can even drop two of the same key.
Key drop odds per difficulty level:
Patch 2.2 changed all key drops to apply globally, thus each Keywarden killed will drop a key for every player (of at least level 60) in the game, regardless of that player's proximity. Keywardens can thus be split farmed with players spreading out into different acts, and letting each other know if each KW drops a key, or not. When one does, players can teleport to the one who got the kill, since keys do not show on the map as legendary drops they are hard to find without a guide.
- Which keys key wardens drop - Blizzard CM, 02/07/14