From Diablo Wiki
The Infernal Machine is an end game quest feature introduced into Diablo 3 in the v1.0.5 patch. This multi-stage event requires players to find keys from special Keywarden monsters. The keys are required to craft an Infernal Machine, which can be used to open red portals to special dungeon areas where Ubers, super powered versions of certain quest bosses, must be defeated. These Ubers have a chance (increasing with difficulty level) to drop a Demonic Organ.
This event and all the materials required for it are Torment only and can not be glimpsed on lower difficulty levels. The basic actions required to obtain the keys and make the Infernal Machines are identical in Diablo 3 and Reaper of Souls. RoS just adds a 4th key and a 4th Uber, upping the complexity slightly.
- Diablo 2's end game event. is the obvious inspiration for this
 Infernal Machine Walkthrough
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.
- Diablo 3: Keys found in Acts 1-3 with the Infernal Machine plan and/or a random key from the Act 4 keywarden.
- Reaper of Souls: Keys from Acts 1-4 in RoS, with one key from each Keywarden.
- Craft the keys (one of each required) to create an Infernal Machine.
- Enter the Hermit's Shack in Act One Tristram town.
- Activate the Infernal Machine to open a red portal to one of the Realms.
- There is only one kind of Infernal Machine in D3, and the portal opened is random. Players are advised to have 3 Infernal Machines to do all three in the same game.
- There are 4 types of Infernal Machines in Reaper of Souls, so players can open precisely the portal they wish.
- Slay the Ubers in the realm, and cross your fingers for a Demonic Organ to drop along with the other loot.
- Once you have all 3 Demonic Organs (4 in Reaper of Souls) visit the Jeweler to craft them into a Hellfire Ring.
It is usually necessary to repeat the initial steps multiple times, since keys and organs do not drop every time. Even on Torment 6 they have only a 50% chance to drop. Players can farm for the keys and organs much more efficiently in multiplayer games, both for the faster killing speed and since every player in the game has a chance at each item dropping, and all characters in the game can pass through the same portal(s) to the Uber battles.
 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:
Prior to D3v2 key and organ drop odds were 5% even on Normal difficulty, and could be raised all the way to 100% on Monster Power 10.  It was also necessary to have five stacks of Nephalem Valor, though Magic Find did nothing to improve the drop odds.
The four keywardens:
- Odeg the Keywarden in Act I - Fields of Misery.
- D3: The Key of Destruction. RoS:
- 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
The keys, plans, organs, and machines are all Bind to Account so must be collected by each player individually.
 Building the Infernal MachineBlacksmith. He needs to be level 10 in Diablo 3 (Level 12 in RoS) to craft Infernal Machines. Take the IM into the Hermit's Shack and activate it there to open a red portal to one of the Realms.
 Diablo 3 Realms and Ubers
In Diablo 3 there is only one kind of Infernal Machine and it opens a random portal. Players are advised to do 3 machines in the same game to be sure they get all three portals. Each Realm must be cleared out before the next portal can be opened.
- The Realm of Discord. Ubers: King Leoric and Maghda. May drop the Writhing Spine.
- The Realm of Chaos. Ubers: Ghom and Rakanoth. May drop the Devil's Fang.
- The Realm of Turmoil. Ubers: Siege Breaker and Zoltun Kulle. May drop the Vengeful Eye.
 Reaper of Souls Infernal Machines
In Reaper of Souls players can make 4 different types of Infernal Machines, each of which produces a known Portal, making it possible to hunt each Demonic Organ specifically.
- Infernal Machine of Bones > Realm of Discord. Ubers: Leoric and Maghda, may drop Leoric's Regret.
- Infernal Machine of War > Realm of Turmoil. Ubers: Siegebreaker and Zoltan Kulle, may drop Idol of Terror.
- Infernal Machine of Gluttony > Realm of Chaos. Ubers: Rakanoth and Ghom, may drop Vial of Putridness.
- Infernal Machine of Evil > Realm of Fright. Ubers: Uber Diablo + summons random Ubers, may drop Heart of Evil.
 Hellfire Rings
The main perk of the Hellfire 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 D3 Hellfire Ring on a Follower is worth 7% more Experience to the player, and a RoS HR is worth +9% Experience.
- The HR is account bound and can not be sold or traded.
- The Hellfire Ring is unique equipped so that each character can only wear one Hellfire Ring at a time, whether the D3 or the RoS type.
- Initially during the RoS beta it was possible to wear an imported D3 version along with a new RoS version, but this was a bug.
- Your Followers can wear one also, but not two.
- There is no level requirement on the ring making it an ideal twink. Any level of character or Follower can wear one.
 Diablo 3: Hellfire Ring
Squirt the Peddler sells the Designs that teach the Jeweler to craft Hellfire Rings. In Diablo 3 there are 4 Hellfire Rings, 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. Crafting the ring costs another 50,000 gold.
D3: Hellfire Ring:
- Primary Affix:
- +300-329 to one Attribute. (Varies by ring design.)
- Secondary Affix:
- Increase Bonus experience by 35%
- 4 random properties
- Legendary Property: Chance to launch an explosive ball of Hellfire when you attack.
The attribute bonus was +170-200 to one Attribute prior to the D3v2 patch, but that was prior to Primary/Secondary affixes, so all 4 random might roll the equivalent of what are Primary affixes today.
 Reaper of Souls: Hellfire Ring
- Primary Affixes:
- Secondary affixes:
- Increases Bonus Experience by 45%
- Legendary property: Chance on hit to engulf the ground in lava, dealing 200% weapon damage per second for 6 seconds.
- +5 Random Affixes
The RoS Hellfire Ring proc effect is a lava pool on the ground. When the Hellfire Ring was first enabled during the RoS Beta test, the proc was a nova-like burst of fireballs.
 Reaper of Souls Gallery
Screenshots showing various aspects of the Infernal Machine and Hellfire Ring in Reaper of Souls
 Reaper of Souls Video
From the RoS beta, two characters clearing all four Realms and creating the ring.
 Diablo 3 Gallery
Diablo 3 Infernal Machine screenshots.
The Heretic's Abode
 Diablo 3 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