The Night Warrior is the name of Elune's warrior aspect, her fearsome battle aspect. As the Night Warrior, Elune is said to take the valiant dead from the battlefield and set them riding across the evening sky as stars for their reward.
It is also the name given to the person who channels Elune's Night Warrior powers in a ritual to become the avatar of the Night Warrior. This is extremely dangerous, and even witnessing the ritual can be a danger.
The first known Night Warrior was Thiernax, who lived on the planet Fyzandi. Long ago, their planet was attacked by an Old God. The Old God and its minions waged war on the people of Fyzandi for generations, starting in the era of Thiernax and his husband Qadarin's great-grandparents: this conflict became known as the Endless War. Qadarin's mother attempted to become the Night Warrior to stop it, but the power killed her. Thiernax underwent the ritual as well and managed to obtain the power to defeat the Old God. However, the power of the Night Warrior was destroying him, so he shared some of his power with Qadarin. This still wasn't enough to contain Elune's power, and they died, having saved their world.
Legend says that long ago, the Night Warrior was used by the night elves to carve out their empire. None who have attempted the ritual since then have survived, for Elune's raw power tears them apart.
During the Fourth War, Tyrande Whisperwind survived the ascension to become one. Not only was she empowered, but many other night elves were as well, forming the Army of the Black Moon. Elune's power also shrouded Darkshore in an unnatural permanent night.