Long ago, in the early years of the Kaldorei Empire, the use of magic in society brought with it a variety of new skills and concerns. Given the power of the Well and the arcane energies that fuse with life forms living on the shores, those with latent abilities would appear. It soon became apparent that physical combat, a skill inherited from their ancestral race/evolution from the Dark Trolls, wouldn’t be enough.
At the time only a few scattered soldiers and guardsmen were found to possess early magical abilities, such that were minor and untested at best. From these unique forms, numerous organizations and clubs began to rise up, defending the populace and further enhancing their skill sets. Soon, spellcasters of great potential rose with the ambition to be noticed for their aptitude and magical talents. One group in time gained sanction from the Monarchy to protect the Empire and serve the populace.
This group became known as the Moon Guard, aka the Guard for short.
In time, they built a mighty stronghold in the mountains a great distance from the capital of Zin-Azshari but close to aid in the defense of Suramar. If Zin-Azshari was the crown of the Kaldorei Empire, Suramar was its greatest Jewel. Within the region, the natural ley lines were charted and rerouted to aid as a source of mana for the Stronghold; second only to the power of the Well of Eternity itself.
For their unique skillset with arcane, the Moon Guard became loyal only to the monarch of the realm. Members soon regarded lesser spellcasters with an inferior air about them for they did not belong on the same level as the Guard. They enhanced their talents and hoarded both great knowledge and arcane relics to further their latent and hidden talents becoming a group of powerful sorcerers indeed.
Inbreeding and physical alterations created unique casters that caught the eye of Queen Azshara herself. They became known as her elite Highborne, nobles with unlimited arcane potential. Even the Moon Guard was seen as inferior to this caste and were shunned, yet they still remained loyal to her majesty. Many believed they lost favor not because of the Queen but because of her most loyal counselor Xavius, who uses his resources and knowledge to manipulate royalty. Still, they studied and continued their duties to ensure the safety and well-being of the Empire.
It was during the events of the War of the Ancients that many within the Guard began to question their allegiance having seen the changes and rise in demons among the populace. The people were scared that their once great empire had been compromised. The Moon Guard thus severed ties with the Monarchy and Highborne, joining the resistance and allying under the leadership of Lord Kur’talos Ravencrest. Many members of the Guard were separated and joined in the defense of Black Rook Hold under the leadership of Illidain Stormrage, while others remained in defense of Suramar alongside Jarod Shadowsong.
It was during the cataclysm caused by the implosion of the Well of Eternity that the survivors of the Moon Guard found themselves cut off from their allies and the rest of the empire. They found their greatest arcane power was lost and some succumb to arcane withdrawals having been relying greatly on the well during the war. It was then that the survivors among the populace took action and condemned the study and use of arcane magic. Most of the organization disbanded having felt great loss during the war, renouncing the sorcery that had once been their primary purpose.
Some of them turned instead to druidism, seeking to redress the imbalance their magic had caused. These former sorcerers consequently became some of the first students of Malfurion Stormrage. Those that remained chose isolation and barred themselves within the Moon Guard Stronghold choosing exile and to resume study than forsake their gifts. Unlike other strongholds of arcane study across the empire, the Moon Guard prepared for such an eventually as seen by mystics within. Thus, the use of the ley line conduits as a magical source would sustain the sorcerers for the long years in isolation.
(This part is just conjecture because the presence of Coils in the Stronghold suggest tapping into arcane sources for sustainment. Eldre’thalas used a demon for power, and Shandaral had to steal from the Nexus for sustainment.)
After the events of the sundering of the Well, the Moon Guard continued their research and study into the Arcane. While its neighbors within Suramar City were forever cut off because of their shield, the remaining residents honored the fallen within Tel’anor and watched from afar in case of the fated return of the Legion. The Watchers enlisted potential fighters, where as the Unseen Path recruited and returned to the wilds. The Dreamweaver’s resumed their peaceful watch of the forests of Val’sharah under the tutelage of the Guardian Cenarius.
For 10,000 years the Broken Isles remained separated from the rest of Azeroth. The Moon Guard maintained its position as protectors of the Kaldorei addressing any magical concerns or incidents in nature that would disrupt the peace. It was one such eventful evening that those within the stronghold bore witness to the collapse of the shield surrounding Suramar City. They attempted to make contact and were greeted by their brothers and sisters over the long years in isolation. The Nightborne as they now called themselves, were seen as a change to the long quiet.
For a moment peace seemed to quell the silence of several centuries but this ended after a brief sigh of respite. Several squadrons of these Nightborne teleported within the Stronghold and attacked without pause or mercy. The Moon Guard was caught unaware of any reason for this provocation and could not react quick enough. In a few short hours, many within were slaughtered and the survivors did their best to barricade or defend themselves within the halls. It was only when outlanders appeared that the tide turned.
Adventurers of unknown origin or race appeared and defended the survivors. A strange city appeared in the sky floating on a rock that led these heroes to this land and they fought with the skill and determination of seasoned fighters. Survivors of the Guard were able to escape, some ported away to nearby settlements such as those within Val’sharah hiding among allies. Others teleported to a nearby settlement of nightfallen exiles all in various stages of arcane withdrawal. They allied with these outlanders and together pushed back against their foes to the center of the Nightborne city, Suramar.
After meeting retribution at a combined force of Kaldorei, Nightfallen and even strange light skinned elves of similar alteration called Blood Elves, was the Moon Guard was able to take a reprieve to honor the dead and reacquaint themselves with their kin albeit for a time. The light skinned elves and the Nightborne chose to join with an opposing force known as the Horde, where as with surprise and a sense of admiration, the Moon Guard was welcome back into Night Elf society; as they became known to the outlanders.
For a time, the survivors of the Moon Guard restrained themselves to their stronghold, pausing to wonder if they were deserving of the opportunity to rekindle their connection with their people. But now long after the suffering caused by the Horde and the rebirth of a new home for their people, does the Moon Guard take the hand of kinship and seek to rejoin and rebuild their force. Thousands of years’ worth of history, both arcane and tradition, remain within the grasp of new acolytes and mages seeking to rebuild/rekindle a lost legacy.
(This is just a rough history from what i gathered across Moon Guard, Highborne, Night Elf, Suramar, and other pages of Lore. There in lies a deep history of the Guard and a history of magic for those that love this area of lore.)
to get that name? 
