The main draw of the necklace for a level-70 character is the passive skill and the socket it automatically creates, allowing you to slap in a legendary gem.

The Hellfire Amulet itself is a worthwhile creation but you may have to end up farming Keywardens and grinding the various bosses inside the four portals in order to get the passive skill you want on it. It should be pointed out that the level-70 version of the Ring is generated using the Smart Loot system, so it may not be optimal for the class you intend to use it on, but even if it has bad stats for your alt, that 45% XP bonus is still pretty amazing. Since the Ring and the Amulet require the same materials to craft the level-70 version, we’ll focus on the Amulet from here on out. If you’re trying to power level an alt, the Hellfire Ring is one of the best options. The Hellfire Ring is great for your alts, due to its XP bonus - a level 70 Hellfire Ring can be equipped by a level 1 character and grants a 45% XP bonus, the best in the game. What do the Hellfire Ring and Hellfire Amulet do? Luckily, I’m married to someone a lot smarter than I am, and so eventually I was clued into the fact that I could use those self-same Infernal Machines to craft the Hellfire Ring or Hellfire Amulet, and the Amulet in particular is a really good item for a neck slot item for even an experienced character. So for a long time, I was just accumulating Infernal Machines in my bank for no reason.

I mean, they were there, they were monsters, it was Diablo 3 - I didn’t really need a reason. I spent a lot of time killing the Keywardens in Diablo 3 and not knowing why I was doing it.

Diablo 3 infernal machine guide