After freeing Garona and witnessing Gul'dan's takeover of the Iron Horde, the Tanaan leg of the journey begins.
The "wait for things to drop" part of this step - the one that will take the longest - requires you to collect 33 Tomes of Chaos from Hellfire Citadel. Unlike Abrogator Stones and Elemental Runes, Tomes are not a guaranteed drop, so you'll sometimes have frustrating weeks where you only get a couple. I actually preferred the Abrogator/Elemental Rune model, with the guaranteed drops coupled with the occasional bonus. But after enough time, you'll get them.
Meanwhile, you'll have to use the Shipyard. You'll get a series of naval missions that show up on occasion. There's not a lot to these - just complete each mission as it pops up, and you'll eventually capture Gul'dan's flagship, the Master's Call. No, you don't get that as a ship for your shipyard. It's kind of anticlimactic.
Upon collecting the tomes, Khadgar will tell you to take them to Cordana Felsong, who you'll task with destroying the books. You can't simply burn them, as there are actually innocent souls bound to the pages... so that's horrifying.
Anyway, you arrive in Zangarra and discover that Cordana has set the tower off-limits. You go and talk to her and she's obsessively staring at the Shadow Orb you had captured in Shadowmoon Valley. She desperately asks you to give her your ring, which you can either simply refuse or respond "Oh HELL no!" after which she will attack you. You need to destroy the tomes she has set up around her (sorry, innocent souls) and then fight her - which took a while, but I was in Prot spec.
When her health gets low, Cordana escapes through a Nether portal, going to her new master, Gul'dan (I assume she's been mind-controlled just like Garona had been.)
You return to Khadgar, who is understandably unhappy with this turn of events. He also realizes that the last couple times he has upgraded your ring, he's accidentally killed you. He also doesn't really have any more power to imbue you with, so he sends you elsewhere. Alliance players go to Karabor to receive K'ara's blessing (I assume Horde goes to somewhere in Frostfire to get the blessing of the Furies or something.)
You meet with Yrel and the other Exarchs, and K'ara blesses you. But it's not K'ara who speaks with you - instead, it's the spirit of Velen. No one else hears his words, so I think what this means is that Velen has merged with K'ara to become a Naaru himself. Not a bad fate for the old Prophet.
With your ring finally legendary, you are sent on one last mission - to kill Gul'dan.
Of course, that doesn't actually work out, as Archimonde saves him after you defeat the Defiler (and sets up Legion.)
The Legendary Rings have some interesting mechanics. The one I've actually used (in the fight against Archimonde this week) is the tanking one. On a two-minute cooldown, shared with all of the other tanking rings on other tanks in the raid (so, with your co-tank,) you both get 50% Versatility, and all damage you take is split evenly between the two of you. So you essentially get a 50% damage buff, a 25% incoming damage reduction, and a kind of Spirit Link Totem between the two of you.
It's a pretty powerful buff, especially given the rather short cooldown.
Overall, I think I liked the Mists of Pandaria legendary chain better. There was more to it (granted, there was more to Mists of Pandaria,) and a lot more lore development. I like Khadgar a lot, but ultimately, this didn't feel as exploratory or revelatory to the expansion. Also, there's no visual to the Ring except when it is used, which is really underwhelming after the cool wing/wreath/thing that popped up with the Legendary cloaks.
I also think that it was smart to let players get the cloak before getting into Siege of Orgrimmar. As it stands now, unless my guild starts raiding again, I'm basically just going to use this ring for leveling up in the Broken Isles, as I exhausted the gear that I need out of HFC LFR a while ago, as well as that of the Tanaan jungle dailies (a mix of Demonbreaker set pieces from LFR and 695 Baleful bits - I guess I could use a couple drops off of Kazzak.)
As it stands, the quest chain kind of reinforces one of the problems with the lore in Warlords - the only long-term effect on WoW is that Gul'dan will be back (long enough to set off Legion and die in the first raid tier.) We lost Maraad, yes, but he hadn't been a presence in the game until Warlords.
I do wonder about Legion's take on legendaries. In a sense, artifact weapons will be legendary as soon as we get them. I think they've said that the legendary chain in Legion would probably focus on those weapons. It might not even make sense for Legion to have a legendary chain (maybe they can take that effort and focus it more on quests for the various class orders.)
I also wonder if perhaps this "whole expansion" legendary quest chain model is, ultimately, a good one. It makes catching up if you started it late extremely time-consuming. I also think that perhaps it's all right if legendaries go back to being something only a few people can do. Anyway, we'll have to wait for a while to get more details on how it'll work in Legion, but the main point is: I have my legendary ring. Hooray!