Quick Background
The Nelves of Suramar used the power of the Well of Eternity during the struggles of the War of the Ancients to erect an impentrable barrier that sealed and hid most of their city. Before that happened, they used the Pillars of Creation to seal a huge portal the demons had opened in the HQ of the Order of Elune (shortly after the last High Priestess (Tyrande) was commissioned after her predecessor died). It came at a time where there most famous citizens Tyrande Whisperwind, The Stormrage Twins, and the Shadowsong Siblings with a group of NElves from Suramar had started the main resistance to Azshara and her legion allies.
We know the story of how Kur'talos Ravencrest joined the fight, and the Night Elves from this region and largely this city waged a war to Zin'Azshari. While they were thwarting the plans of Azshara and Sargeras en route to the Glory of Azshara, those remaining in the city of Suramar, closed off a demonic portal that could quite possibly have over-run the resistance army and certainly the city. In the final moments before the Sundering, Ellisandre and the Night Elves of the city drew on the power of the Well of Eternity to shield and hide their city as the rest of the world as they know it ceased. It is conceivable that it is because of their effort that the borken Isles remained as an island while all the other land around it plunged into the depth of the sea
The Nightwell
So, one of the interesting features of the nightborne NElves is the nightwell they created using the Eye of Aman'thul, that essentially allowed them to survive starvation that was happening. However over time the font has become corrupted in a "it's broken" sense rather than an "old god shadowy evil" sense or an "evil corrupt fel-power sense"
After they sealed the demonic portal, before the City was sealed up, the Pillars were hidden, but it is possible that either the process used to seal the portal or divide the pieces, or maybe using one of the pillars to such a large extent without the others caused a flaw, corrupting the flow in time. We're not given the full detail, except that the Eye of Aman'thul was used to convert the rich leyline nexus of energy beneath Suramar into an enhance well of arcane power called the nightwell. And over a long period of time it has become corrupted, like a damaged flow - it's not working like it should do, and this causes nightborne nelves who don't use it to eventually become a horrible mindless state "Withered" before dying. Pretty much like you will starve without food/water after a while and die, but this has the extra kick of a mindless phase.
It's the opposite of the high/blood elf situation where they indulged in the sunwell, and the subsequent arcane corruption that results from over-indulgence of arcane energies to feed their hunger pangs during the absence of the sunwell resulted in them becoming Wretched. Another key difference is High/Blood elves had a choice over their issue, not restraining themselves (something they at least could do as many BElves & HElves showed) leads into that cursed state, whereas the nightborne nelves can't choose not to wither if they don't sustain themselves by this energy.
What Happens During Legion
So we have an interesting state. We know the Nightborne NElves in normal state aren't any more good or evil than anyone, making choices they do for their own reasons. Some like Ellisandre their leader chose to align their society with the Legion, others like First Arcanist Thalyssira and the noble Lilleth remember they were once noble and fight. Most nightborne however are terrified of been cast out, so tow the line that is getting them enslaved and harvested by the legion for soul shards. The majority of the player campaign is assisting the night elves of the area, the first you meet is the Nightborne Nelf Thalyssra who explains some of what happens and directs you all over the zone where you meet and gather other Nightborne NElves willing to fight and normal night elves too who have been devastated by the works of the Legion and their allies.
Your campaign leads to the Assault on the Nighthold that will eventually help the resistance overthrow Ellisandre and defeat Gul'dan - which will lead you into patch 7.1 and presumably the Tomb of Sargeras raid that will involve Illidan, first as foe then as friend.
What Might happen AFter Nighthold
The big question though, is what will happen to the nightwell - will blizzard destroy it like they did the Well of Eternity? or will they save it like they did the Sunwell?
What are your feelings on this? Once we win, I can't see the Night elves doing nothing about the nightwell, For the first time since the sealing of the portal 10k years ago, the pillars of Creation have been re-united. Furthermore, normal night elves specialities are cleansing arcane Well fonts of power - this is what they do in the moonwells.
I'm thinking if blizzard doesn't destroy the Nightwell, I'm thinking that cleansing will happen via Elune and the High Priestess, and involve the pillars of creation - cos unless they utterly destroy the nightborne, they're going to have to some how help them out of this.