From Diablo Wiki
The Infernal Machine is an end game quest feature introduced in the v1.0.5 patch. This multi-stage event allows players to find keys, which can be used to open red portals to special boss fights. Those bosses can drop special monster organs, which can be crafted into the legendary Hellfire Ring.
This event is only functional on Inferno difficulty and is only viable for very strong characters. It's meant to provide a fun bonus late game feature for expert players, adding additional challenge and farming options.
 Quick Summary
Here's a quick summary of the steps in this event. See the sections below for full details about everything related to the Infernal Machine.
- Obtain a key from each Keywarden and the Infernal Machine plan.
- Use the keys to open a red portal to one of the three Realms.
- Slay the special uber bosses in the Realm to earn one of the three demon organs.
- Craft all three organs into a Hellfire Ring.
Note that steps 1-3 must be repeated multiple times, since you must create at least three red portals to obtain all three demonic organs. Furthermore, unless you are playing on Monster Power 10, the keywardens do not always drop keys and the uber bosses do not always drop the demonic organs.
 Find the Keys and Plan
The first stage in unlocking the Infernal Machine players have to gather 3 keys and a plan from all four acts by killing the Key Wardens. (An even longer and more involved process than Diablo 2′s.)
The Keywardens appear in their designated areas only on Inferno difficulty. (They were found on all difficulties prior to v1.08.) If a player has 5 stacks of Nephalem Valor the keywardens may drop a key. The only way to increase the odds of a key dropping is to increase the level of Monster Power.  
Key drop odds per Monster Power level:
- Monster Power 0: 5%
- Monster Power 1: 10%
- Monster Power 2: 20%
- Monster Power 3: 30%
- Monster Power 4: 40%
- Monster Power 5: 50%
- Monster Power 6: 60%
- Monster Power 7: 70%
- Monster Power 8: 80%
- Monster Power 9: 90%
- Monster Power 10: 100%
The four keywardens and the planwarden are:
- The Key of Destruction will drop from Odeg the Keywarden in Act I - Fields of Misery
- The Key of Hate will drop from Sokahr the Keywarden in Act II - Dahlgur Oasis
- The Key of Terror will drop from Xah'Rith the Keywarden in Act III - Stonefort
- The Blacksmithing Plans and random keys will drop from Nekarat the Keywarden in Act IV - Silver Spire Level 1
|Key of Destruction||Key of Hate|
|Odeg in the Fields of Misery, Act 1||Sokahr in the Dahlgur Oasis, Act 2|
|Key of Terror||Infernal Machine Plan|
|Xah'Rith on the Stonefort, Act 3||Nekaret in Silver Spire, level 1, Act 4|
The Keys and Plan are Bind to Account so you can not trade for them. You can have more than 1 of each key at a time, but each key is consumed by the crafting process, so you 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.
 Building the Infernal MachineBlacksmith. He needs to be level 10 to craft the machine recipe, and this costs 12,000 gold per craft. This creates a Portal Device which appears in your character's inventory and can be carried around until you decide to use it.
To use the device join a game at the Act 1 quest Return to New Tristram (db) and locate Brother Malachi the Healer in the north east of Tristram. The house behind him has a boarded up door which is normally unclickable. With the Portal Device his door will light up as clickable and lead into the Heretic's Abode.
Once you're inside the house, right click the Portal Device in your inventory. This spawns a red portal that will take you to one of the three special Realms where you must battle two Uber bosses at once. Upon death, one of them *may* drop the special monster organ which is required to craft the Hellfire Ring.
As with the keywardens, you *must* have the maximum 5 stacks of Nephalem Valor active when you kill the bosses to have any chance of earning their monster organ. Also, killing them on a higher setting of Monster Power will increase the odds of getting the organ by 10% per Monster Level, with a 100% chance on the maximum Monster Level 10.
Each realm contains two bosses you must fight at the same time. These bosses are all familiar monsters from elsewhere in the game, but in this battle they are greatly buffed in power and boast some new abilities. Blizzard estimates that they are approximately as difficult as Diablo himself, on Inferno difficulty.
- The Realm of Discord - King Leoric and Maghda.
- The Realm of Chaos - Ghom and Rakanoth.
- The Realm of Turmoil - Siege Breaker and Zoltun Kulle.
The machine will only work once and open one portal. To open another portal you need to reconstruct the machine again, by collecting the three keys, visiting the Blacksmith and paying him 12,000 gold to craft you another Infernal Machine device.
The special uber bosses drop loot like normal elites, and also have a chance to drop the monster organs that are required to craft the Hellfire Ring.
- The Writhing Spine drops from King Leoric or Maghda.
- The Devil's Fang drops from Ghom or Rakanoth.
- The Vengeful Eye drops from Siege Breaker or Zoltun Kulle.
 The Hellfire Ring
Squirt the Peddler sells the Designs that teach the Jeweler to craft Hellfire Rings. There are 4 such Designs, one for each type of attribute and each sells for two million gold. Purchase a plan and right click on it while interacting with the Jeweler to teach him that recipe.
To craft the ring a character must have 50,000 gold and one of each of the Demon Organs in their inventory.
He can craft one of four varieties of the ring where one of the affixes is guaranteed to spawn with +170-200 to a single attribute, either Strength, Dexterity, Intelligence or Vitality. The full ring stats are as follows:
- +170-200 to one Attribute.
- Increase Bonus experience by 35%
- Chance to launch an explosive ball of Hellfire when you attack.
- 4 random properties
- You can only wear one Hellfire Ring. Your Followers can wear one also.
- There is no level requirement on the ring so any of your characters can wear it.
The main perk of the ring is the substantial increase to Experience gain, and the goal for most players is to create a ring that's got decent stats as well, so they're not giving up too much DPS by using the Hellfire Ring instead of a Legendary or Rare ring. Followers can equip the Hellfire Ring as well, but just as with Magic Find and Gold Find on followers, they only share 20% of the bonus experience to the player. Thus a Hellfire Ring on a Follower is worth 7% more Experience to the player.
 Infernal Machine Videos
These two videos show how to find the various Keywardens.
 The Infernal Machine Teased
The first word of the Infernal Machine came in a Patch 1.0.5 Sneak Peek developer blog post on September 13, 2012 . The brief quote is below:
The Infernal Machine is a device that will allow level-60 players to battle “uber” versions of some of Sanctuary’s most nefarious bosses. While the rewards for defeating these bosses will be great, some assembly is required.
- ↑ Drop rate at MP0 - Blizzard CM, 17/10/12
- ↑ Innate drop rates equal - Blizzard CM, 22/10/12
- ↑ Magic Find effects - Blizzard CM, 17/10/12
- ↑ Salvaging the ring - Blizzard CM, 05/10/12
- ↑ Blog Update - Blizzard, 13/09/12
- ↑ Patch 1.0.5 Sneak Peak - Blizzard CM, 13/09/12
- ↑ Interview - IGN, 17/09/12
- ↑ Interview - Gnn.Gamer.com.tw, 18/09/12