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 players have to gather the keys. The four keywardens are located randomly somewhere in their levels, and may require some searching to locate. Remember that this entire event is Torment-only and the keywardens may not appear on lower difficulty levels, and will never drop a key below Torment difficulty.
Key (and Demonic Organ) drop odds per difficulty level:
In D3v2 and RoS, all Keywardens have one key they usually drop, but they have a small random chance to drop additional keys of any type. They might even drop two of the same key, from time to time.
 Diablo 3
- The Key of Destruction may drop from Odeg the Keywarden in the Act I Fields of Misery.
- The Key of Hate may drop from Sokahr the Keywarden in Act II - Dahlgur Oasis.
- The Key of Terror may drop from Xah'Rith the Keywarden in Act III - Stonefort.
- The Infernal Machine Blacksmithing Plan (and random keys) will drop from Nekarat the Keywarden in Act IV - Silver Spire Level 1.
 Reaper of Souls
- 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 - Silver Spire Level 1
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.