From Diablo Wiki
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