War between the titans and the Old Gods

The war between the titans and the Old Gods was a conflict fought eons ago by the titan-forged against the armies of the Black Empire on the main continent of Azeroth known as Kalimdor.

Prelude
When the entities known as the Old Gods plummeted down from the Great Dark and established their Black Empire, the Elemental Lords, who ruled over Azeroth, saw the invaders as a threat to their dominion. For the first time in Azeroth's history, the elements worked together against a common enemy. However the Black Empire, through sheer numbers, would ultimately win the war and enslave the elemental lords and their armies.

Some time after, the titan Aggramar would discover Azeroth and, horrified by the presence of the Old Gods, and unwilling to leave the nascent titan to be corrupted by the Void, the Pantheon vowed to save Azeroth before it was too late.

The war
Unwilling to take action directly due to their colossal forms and risk of damaging the world-soul, the Pantheon created an enormous army from the crust of Azeroth itself, called the titan-forged. In response, the Old Gods called forth the Elemental armies into battle. With the Elemental Lords and their minions besetting the titan-forged from all sides, the Keepers decided to divide and conquer them.
 * Tyr and Odyn fought Ragnaros the Firelord for weeks, and through force of will the pair defeated the Firelord
 * Archaedas and Freya unleashed their powers upon Therazane the Stonemother and forced her into retreat before submitting her
 * Ra, Thorim, and Hodir faced Al'Akir the Windlord and ultimately turned the Windlord's own power against him
 * Neptulon the Tidehunter was beset by Loken and Mimiron, who would shatter his army

Though the elementals were defeated, the Keepers knew it was only a matter of time before they reformed, and so Ra used The Fist of Ra-den to forge the Elemental Plane, a prison to which he would bind all the elementals they defeated. The titan-forged then decided to focus on the monstrous Old Gods. Y'Shaarj, however, was more powerful than they had expected and, concerned that the Old God would overwhelm their servants, the titans decided to take direct action. Aman'Thul reached out and violently tore Y'Shaarj from the crust of the world. Though Y'Shaarj was dead, the titans had discovered that the Old God had buried himself too deeply into Azeroth, as they beheld the life energies of the world-soul erupting from the newly created scar within Azeroth.

Odyn thus suggested to try and imprison them, rather than risk damaging Azeroth further. N'Zoth was the first Old God to be locked up deep beneath the earth, followed by C'Thun who was imprisoned near what would later become Ahn'Qiraj. The cunning Yogg-Saron however, the most powerful of the remaining Old Gods, unleashed the mighty C'Thraxxi generals, whose mere appearance whipped the remnant Black Empire forces into a frenzy, upon their enemies. Odyn commanded Loken to use a spell to trick the Black Empire armies into seeing one another, and even Yogg-Saron, as their enemy. With the Black Empire warring against itself, the titan-forged were able to pierce through the confused mass and imprison Yogg-Saron in the far north of Kalimdor, near what would later become Ulduar.

Aftermath
With the Black Empire defeated, a tentative peace settled on Azeroth. The Keepers would urgently craft magic wards around the gaping wound left behind by Y'Shaarj to stanch the lifeblood of the nascent titan. Eventually, the seeping energies would calm and settle into a lake of arcane energy that would become known as the Well of Eternity.

When Aman'thul destroyed Y'Shaarj, its remains fell back on the land that would one day become known as Pandaria. Highkeeper Ra discovered them, alongside the Heart of Y'Shaarj, and sealed them inside the Mogu'shan Vaults, seeking to study them in order to learn the nature of the Old Gods.

The titan-forged races were tasked with shaping Azeroth, while Freya began to seed life. Over time, plants, trees, all sorts of creatures of every kind began to thrive on the primordial continent. As twilight fell on the final day of their labors, the Titans and their followers named the continent Kalimdor, "Land of Eternal Starlight".

Trivia

 * With different sources of information such as the original backstory and the Tribunal of Ages, there was confusion on which between the titans and Old Gods arrived first on Azeroth. Chronicle Volume 1 however established that the Old Gods were first to discover Azeroth.
 * Soggoth the Slitherer, Warlord Zon'ozz and Lord Rhyolith fought on the side of the Black Empire during the conflict, while Kronn was on the side of the titan-forged.

Speculation

 * Xal'atath mentions an event called "the Scouring" in her quote whisper It likely refers to the fall of the Black Empire and the subsequent scouring of the aqir and n'raqi by the titan-forged forces.
 * What the vrykul call the "era of Shadow" is likely the reign of the Black Empire.