Spoilers for Wolfheart in the post below.
The Wardens, watchers, jailers, guardians of Night Elf society for 10,000 years, have been through much in the past decade. From once-trusted former members of the Temple of Elune, they took up the supposedly eternal duty of guarding their race's greatest traitor, only to be killed by their own leader for performing the duty appointed to them. The survivors of Tyrande's attack then found themselves buried alive in the crumbling remains of Suramar as it was collapsed by Illidan, their former captive and target of their manhunt.
When Maiev Shadowsong returned from her captivity in Outland as a prisoner of the same Betrayer she and her Wardens had spent millennia holding themselves, she rebuilt the order. This renaissance for the Wardens was short-lived; Maiev's new Watchers were loyal to Maiev first. Despite the results of Maiev's actions and her abrupt departure from Night Elf society, there was still a need for Wardens when the forces of Ragnaros the Firelord made war on Mount Hyjal.
Now the Shadow Wardens, led by Saynna Stormrunner, attempt to make up for Maiev's betrayal and help lead the defense of Hyjal by taking part in the counteroffensive that brings the war to the Firelands' Molten Front. For an organization with so long a history, to be nearly wiped out twice and find your loyalties cast into question rankles. Yet the Wardens endure for one reason: Someone has to be the dagger in the shadows.
They watched forever
If not for Illidan, of course, there never would have been Watchers. After the War of the Ancients and the destruction of the Well of Eternity, Illidan was brought to trial by his brother Malfurion. While much of Kaldorei society had viewed the destruction of the Well and the loss of the arcane power it provided their people as a necessary evil in order to prevent Sargeras and the Burning Legion from entering Azeroth in such force as to be unstoppable, Illidan could see only the consequences of such an action. He could not abide them.
And so, Illidan crept to the summit of Mount Hyjal and poured vials of water from the original Well into the lake atop the mountain, creating a new Well. In so doing, Illidan presented the survivors of the War with the horror of such a conflict one day resuming. Illidan's action led to the dragon aspects Alexstrasza, Nozdormu and Ysera's making the ancient pact with the Night Elves, granting them immortality in return for eternal vigilance over the Well and the World Tree Nordrassil that would grow to obscure and protect it. They also led to a conundrum: What to do with Illidan himself?
Rather than execute his own brother, Malfurion chose to imprison him. Former priestess of Elune Maiev Shadowsong had proved able to keep Illidan in check during the period before his trial, and so Malfurion asked her to select volunteers to create a force that could guard his brother for the duration of his imprisonment. Since the pact with the aspects meant that imprisonment would last eternally, these volunteers would essentially be consigned to the role of Illidan's watchers for all eternity. Maiev, disenchanted with the priesthood for various reasons (including jealously that Tyrande, and not her, was selected by Elune to be High Priestess), took up this task with grim resolve and exceeded anyone's expectations.
Despite their personal antipathy for each other, Tyrande, as leader of the Night Elves, recognized that the organization Maiev had built was a useful tool as their people rebuilt and changed following the War. New threats such as the Satyrs' (those former Night Elves cursed by Lord Xavius, Azshara's most faithful Highborne servant and the first of the satyr who made the rest of the race) calling themselves the Lords of the Emerald Flame would need to be dealt with. The Night Elves needed a force that could deal with such threats. There would always be Kaldorei who felt the call of the arcane and turned to the Satyrs, the Naga, or some other malefic force and these would need to be hunted down, rooted out and imprisoned or killed. While the Sentinels served as the Kaldorei army, the Watchers grew to be its spies, scouts, and jailers.
This continued for many thousands of years, the Watchers fulfilling their ancient duties, their elite attaining the rank of Warden after their leader. Warden Maiev was ruthless, practical, and entirely focused on her task and her order, as it gave her the purpose she lacked after leaving the priesthood.
By Eternity's End
The coming of the Third War completely changed a society that had endured in xenophobia and isolation for many thousands of years. While the War of the Shifting Sands saw the first cracks in the Kaldorei's stasis, it wasn't until the Third War a thousand years later that things finally fell apart. By that time, the Night Elf way of life resembled an insect trapped in amber, perfectly preserved, unchanging, yet unmoving and not truly alive. The shores of Kalimdor were protected from outsiders by the ancient magics of the pact, and if the other people who shared the continent with the Night Elves were warlike and even genocidal (such as the centaur), it mattered little to them.
But the Third War could not be ignored. The Burning Legion had returned, using Orc slaves to invade Azeroth. Even as those same slaves found themselves free from their Legion masters under the leadership of a former slave gladiator named Thrall. The Legion itself had created the Scourge, a new and terrible host of undead under the command of the Lich King, a former Orc shaman twisted into a monstrosity of potent necromantic power trapped in a frozen prison.
A return and a fall from grace
The Legion and the Scourge could not be ignored. And so, Tyrande Whisperwind broke ranks with her ancient lover Malfurion and decided that in the face of the Legion, the Night Elves needed Illidan's aid once more. She led a force of loyal sentinels into the Barrow Dens and demanded that the Watchers hand over Illidan to her. They refused to surrender their charge, as Maiev was not there to command it (despite the fact that Tyrande was Maiev's effective superior, the Watchers had always been loyal to Maiev personally as she hand picked the order and led it for 10,000 years). And so rather than wait, Tyrande killed all the Watchers who stood between her and Illidan, even the Keeper of the Grove Califax. Illidan was released and soon defeated a Legion army in Felwood by consuming the power of the Skull of Gul'dan and transforming himself into a hybrid of Nigh Elf and demon.
Maiev was devastated by this action. Tyrande, who had once stolen the leadership of the priesthood from her, now had killed many of those loyal to her and set their most ancient charge free to wreak havoc. His first action was to transform himself into an even more powerful monstrosity. She rounded up all the Watchers who had not been in the Barrow Den and led them on a hunt to return Illidan to his prison, a hunt that led them to the crumbling Tomb of Sargeras.
There, they fought past Naga and eventually cornered Illidan himself, only to discover that he had intended it all along. Using the power of the Eye of Sargeras, a relic of impossible might, Illidan collapsed the Tomb and buried all of the Watchers save Warden Maiev. Maiev knew that she could escape the tomb but hesitated to leave her Watchers behind until urged to escape and warn Malfurion by her closest lieutenant, Naisha.
While Maiev escaped and ultimately pursued Illidan to the Eastern Kingdoms alongside Malfurion and Tyrande (not a pleasant trip for any of them, considering events) and eventually the Black Temple, the loss of her Watchers and Naisha in particular increased Maiev's loathing for her ancient rival Tyrande and Malfurion, who had constantly showed dangerous leniency to Illidan due entirely to their sibling bond. Her years of captivity by Illidan in Shadowmoon Valley only served to increase her hostility and hatred, even after finally managing to destroy her quarry. Illidan's life had given Maiev something to devote herself to, but his death removed that purpose once and for all.
The Watchers were left shattered by the fall of the Tomb and Maiev's long imprisonment. When she returned from Outland to a chilly reception from Tyrande, she decided to rebuild the order. But if she'd been embittered by the deaths of those she had chosen, groomed and led she was even more disturbed by the return of the Highborne (mages, the very same threat that had led to Illidan's imprisonment), the sudden arrival of the Worgen (mongrel beasts born out of Malfurion's druids, another sign of the weakness of the Stormrage family) and how their once-proud, once-independent people were now part of some sort of Alliance with some of the same races whose coming to Kalimdor had ended 10,000 years of perfect safety and isolation for the Kaldorei. It was not to be borne, and Maiev resolved that the Watchers would not bear it.
Leaderless and purposeless following Maiev's attempted coup, many of the Watchers were dead in her defense. Yet the same tumult and chaos that had led Maiev to her actions also most likely saved the order. The attack of Ragnaros' armies on Mount Hyjal left the Night Elves scrambling for a force that could take action against the elementals without leaving themselves open to the attrition of a front line army like the Sentinels. Furthermore, the Sentinels were also occupied fighting the Horde in Ashenvale and couldn't be spared. Saynna Stormrunner led those Wardens who remained loyal to the Night Elves over Maiev personally to the Molten Front and the defense of their people. This faction renamed themselves the Shadow Wardens and served to guard over Fandral Staghelm until the green dragon Alysra deserted her duties to join Ragnaros and orchestrated his escape.
It now seems likely that the Shadow Wardens and those still loyal to Maiev will come into conflict, pitting former Watchers against one another. So are the shifting alliances of this chaotic modern age, when immortality is lost and thousands of years of tradition come crashing down. The Watchers are dead, it seems, even as they stand silent in the shadows, the dagger in the darkness set against the current flames.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.