War between light and shadow

“Even now, the true battle between the forces of Light and Darkness approaches. We will all be called to join, and in the face of this conflict, all mortal suffering will be meaningless.”

— The Prophet Velen

The war between light and shadow is a final, apocalyptic conflict between the Light and the Void that will supposedly take place on Azeroth in the near future, according to visions seen by the Prophet Velen and the emperor of the Arathi Empire. The Arathi refer to the conflict as Renilash.

First mentioned in Cataclysm, the war will feature as one of the plot points of the Worldsoul Saga. The saga's second installment, Midnight, will feature the Void invading Azeroth and adventurers fighting alongside the forces of the Light to "banish the Shadow forever".

About

Velen's vision

“We must prepare the people of this world to stand together. We must be their example to rally against evil. In service we will awaken them all to form the ultimate alliance against the dark.”
— The Prophet Velen

During the Cataclysm, the draenei Prophet Velen was granted a powerful vision of a grand coalition, consisting of every race of the Alliance and Horde—all armored and armed in glowing weaponry and riding flying mounts of all kinds—along with innumerable dragons and naaru, all led by a human warrior of unclear identity clad in Light-imbued armor with a matching blade. The vision ended with the appearance of a vast and all-consuming shadow, and the army charging to meet it. Driven by this vision, Velen has ever since believed that a great war between the forces of the Light and the Void is fast approaching, and that Azeroth will be its principal battleground. He instructed the draenei to remain on Azeroth, help heal the world of the Cataclysm, and in so doing make its people strong for the future and prepare them to unite into the ultimate alliance against the darkness. For a time thereafter, Velen remained in closely guarded seclusion, not daring to waste his energy on anything save preparing for the final confrontation he'd foretold.

In one possible future, an elderly High King Anduin Wrynn, alongside Velen and the draenei of the Exodar, will lead the Light in a final battle against the Shadow.

Xe'ra

The Prime Naaru Xe'ra believed that the Void was a far greater foe than the Burning Legion, that it would take all the peoples of Azeroth and Outland united to oppose it, and that Illidan Stormrage was a hero destined to defeat it. A vision she granted to Illidan showed him transformed into a champion of the Light, leading the "legions of Light" against gigantic Void creatures on a "terrible battlefield" strewn with "mountains of corpses". It is not known if this war is the same one Velen foresaw. Either way, Illidan rejected this fate when he killed Xe'ra.

Renilash

The Arathi of the Arathi Empire refer to the final battle between light and dark as Renilash. This conflict is the subject of The Song of Renilash, a prophetic text. Chapter 11 of the song reads:

15. The Arathi, guardians of the Sacred Flame, formed the Aegis shield, a constellation of courage against the encroaching darkness. Their resolve, a beacon in the night, as stars began to fade into shadow.
16. The Harbinger's forces, nightmares of legs and claws, spun from the loom of oblivion, skittered across the battlefield. Their movements, a grotesque dance, echoed the discordant rhythm of a world teetering on the brink.
17. The wall, built with the living rock of titans, bore the weight of the conflict. Each stone, a silent witness to the struggle, whispering tales of valor to the wind, stirring the spirits of warriors long gone.
18. The remains of gods, scattered across the battlefield, pulsed with an unseen light. Their echoes, a clarion call, stirred the Arathi, compelling them to fight, to kill for the world's dawn.
19. Amidst the chaos, the forgotten one cried, "When the last star fades, the emperor's crown will weigh heavy with the sins of a thousand years." The stalwart soldiers threw down their spears and wept tears of blood and burning anger.

The Empire's Hallowfall Expedition arrived in Hallowfall when they were following their emperor's "sacred vision" across the Storming Sea to fight in Renilash. They settled in Hallowfall, where they now guard "against the end, against Renilash", though they do not know when their watch will end. Some Arathi believe that the unknown third tribulation they now face (their disastrous arrival to Hallowfall and the Day of Darkness being the first two) may be the "the Harbinger's final onslaught of Renilash". During his attack on the Veneration Grounds, Aelric Leid of the Order of Night tried to corrupt the Eternal Flame and triggered a shift to Beledar's Shadow; he claimed that this was the beginning of Renilash.