There's been a lot of buzz recently about the legendary item upgrades in Legion, that you'll be able to collect from Nighthold, which bump your "old" aka current legendaries to the new 940 item level (which will be the same item level that new legendaries will drop at).
Apparently, to get Touch of a Titan (Essence of Aman'Thul drops from Nighhold bosses), which rewards you with Distilled Titan Essence, you'll need to have finished Illidan's current questline, which may not sound that bad until you remember the 80 Lingering Soul Fragments you need to get, as well as the 10 fragments per week limit.
It seems likely Blizzard will change the requirement when 7.1.5 goes live, but there are several ways for them to do so: remove the 10 Soul Fragments per week cap (or at least remove it once your main has finished that part, so you don't have to wait for weeks on alts), remove that part of the Illidan quest from the requirement or, simplest and best of all, remove the quest's connection to Distilled Titan Essence completely.
But, as Blizzard works in mysterious ways, if you really want to get started on that Essence of Aman'Thul farming as soon as Nighthold opens, you might want to get Soul Prism of the Illidari (and the followup that has you killing Helya) done before the 17th of January.
Now, even if this Illidan quest issue is fixed and you can immediately proceed to Touch of a Titan, this upgrade system will still add a lot of problems to the already not-so-great legendary system, as it will take a while to gather up all those essences to get the upgrade, each Titan Essence only upgrades one legendary, and a lot of people still don't have their BiS legendary.
So, they'll probably want to save the upgrade for when they get it, which may actually never even happen. The whole system hasn't been explained well, or at all (we don't really know the exact drop rates of the essences, this new quest requirement was a bit of a surprise,as well) and it's causing a lot of distress to the community, so let's hope Blizzard come out and clarify all the details about it soon.