From Diablo Wiki
The Writhing Spine is a special quest item that is required as part of the Infernal Machine event.
 The Writhing Spine
- T1 = 25% chance to drop.
- T2 = 28% chance to drop.
- T3 = 33% chance to drop.
- T4 = 38% chance to drop.
- T5 = 43% chance to drop.
- T6 = 50% chance to drop.
Video of the battle from the Realm of Turmoil.
 Obtaining a Hellfire Ring
- Obtain the three keys and the Infernal Machine plan for the Blacksmith.
- 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 will drop from Nekarat the Keywarden in Act IV - Silver Spire Level 1
- Craft three keys, one of each, into the Portal Device.
- Activate the Portal Device inside of the Heretic's Abode to create a red portal to one of the three Realms:
- Take the three demonic organs to Haedrig and craft them 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.