You start at tier1 rifts, which upon completion give access to tier2 rifts which are harder, which give access to tier3 and so on. It's basically similar to PoE map system where you climb from the shitty "easy" maps to the higher ones. That's my understanding of it at least. They're also what constitutes the ladder ranking, I believe, not level/paragon. Some interesting stuff, Hellfire amulets sound cool, even though if it takes an affix it's gonna be hard to actually be worth using since amulets are already loaded with stupid powerful affixes(elem dmg, crit and crit dmg, with main stat being pretty heavy too). If it's a secondary, or even better a separate effect, then they'll enable a bunch of builds or make some lesser builds more viables. If anything it's good utility for every class, being able to run one more passive.