At level 70, you can make use of four passive abilities in Diablo III. With a Hellfire Amulet, however, you can have five.
The fact that Hellfire Amulets have a random Passive Skill attached to them make them very powerful indeed, and they’re also guaranteed to come with a socket, allowing you to place powerful gems in them. You’re not just going to find Hellfire Amulets on your adventures though, you need to craft them.
To craft a Hellfire Amulet in Diablo III, you’re going to need to defeat enemies called Keywardens and collect Infernal Machines. You’ll find them in Acts I-IV, but the ones you need to defeat in order to craft a Hellfire Amulet are found in Acts II-IV. Sokahr can be found in the the Dahlgur Oasis in Act II, Xah’Rith can be found in Stonefort in Act III, and Nekarat can be found in the Gardens of Hope 2nd Tier in Act IV. Make sure you’re level 70 before you head out to defeat them and a purple arrow will indicate their location when you’re fairly close.
At level 70 there’s a good chance for Keywardens to drop Infernal Machines when playing on Torment I through III, but it’s not guaranteed. On Torment IV or above, you’re guaranteed to get at least one, with a 90% chance of getting two at Torment XIII. Once you have at least one Infernal Machine dropped by each of the three Keywardens required – you should have Infernal Machines of Putridness, Terror and Fright – head back to New Tristram.
Behind Haedrig the Blacksmith and just to the right of Myriam the Mystic, there’s an abandoned house. Break its door with an attack and you’ll find you can enter it. Once inside, use each of the three Infernal Machines that you’ve found to open up portals. Each portal will transport you to a fight with multiple Uber Bosses, and upon victory you’ll acquire the materials required to craft a Hellfire Amulet. Before you do so though, you’ll also need the Hellfire Amulet plan. That can be bought from the merchant in front of Myriam’s tent in the town in Act II, and you can also buy the plan for the Hellfire Ring there too if you wish.
Once you have the Hellfire Amulet plan and the materials, use the plan in your inventory to learn it and then go over to the crafting area at Haedrig’s tent. You can now craft Hellfire Amulets for 10,000 gold providing you have the materials required – defeat the Keywardens and Uber Bosses again if you need more. Every time you craft a Hellfire Amulet you get to choose which primary stat it has on it too. Keep crafting them until you get one that is just right!