From Diablo Wiki
The Heretic's Abode is a new area added to the game in Patch 1.0.5, as part of the Infernal Machine event. It is unchanged in the updated Infernal Machine in Reaper of Souls, though in RoS players must open 4 portals within.
The Abode is a single room in a gloomy house, accessed from a boarded up shack in New Tristram, located right behind the Healer. There are no monsters or other events in the house, but it is the only place from which players can activate the Portal Device and create a red portal to the Realm of Discord, Realm of Chaos, Realm of Turmoil, and Realm of Fright (RoS only) where Ubers must be battled and the the various demonic organs *may* be found.
In Diablo 3 vanilla players had to be on the final quest in Act One, with the NPC offering caravan passage to Act Two, in order for the Heretic's Shack to be accessible. This quest status is no longer required in D3v2 or RoS. Note that Heretic's Abode and all related Infernal Machine activities are Torment-only in D3v2, and can not be done on any lower difficulty.
 Obtaining a Hellfire Ring
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 from the Keywardens in Acts 1-3. 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 Heretic's Abode in Act One Tristram town.
- Activate the Infernal Machine(s) 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 at least 1 of each type of Demonic Organs (3 in D3, 4 in RoS) 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.