Breath of Rukhmar
The Breath of Rukhmar was a mighty arakkoa weapon created by the ancient Apexis order that wielded the Light, the Anhar. The Breath of Rukhmar was named after their sun goddess Rukhmar and channeled the energies of the sun into incredibly destructive power.
The weapon was used two thousand years before the opening of the Dark Portal against the Primals led by Gnarlgar and the Sporemound Taala as they marched on Arak. When it was first activated, a violent tremor shook the spire as solar energies roared through it and a white-hot beam exploded from the mechanism to lance through Taala's chest, blowing it apart in a cloud of spores and ash. The Breath of Rukhmar sliced through the botani, genesaur, and Gnarled, incinerating thousands in an instant. The few that survived retreated to Talador in terror, but they were engulfed in flames by the Apexis's Anhari order. The forests that had crept into Arak were seared away. Once the attack was halted, blackened earth and smoldering roots stretched as far as the eye could see.
Centuries later the Breath of Rukhmar was destroyed when the Apexis fell into civil war between the Anhar order and its counterpart, the Skalax which used Void and arcane. The civil war engulfed the spire for many months, and to turn the tide of the conflict the Anhari harnessed the Breath of Rukhmar. As the weapon ignited the Skalaxi leader knew they would be doomed against it and led a handful of his most gifted sorcerers to the top of the spire where they stormed through the Anhari guards. As the Ahhari cut down the intruders the Skalaxi leader weaved a spell to destabilize the Breath of Rukhmar. It worked, but the result was catastrophic: a furious explosion erupted from the Breath of Rukhmar, instantly killing most of the arakkoa on the spire and shattering the land into several smaller spires.
Rise of the Horde
1,200 years after the Apexis fell, a recreated Breath of Rukhmar would come to be known as the crystal superweapon. Created by High Sage Viryx using Apexis artifacts, the superweapon focused the power of the sun to destroy the Adherents' enemies. It was used by the Adherents of Rukhmar to scorch the settlements of their ancient enemies, the Arakkoa Outcasts. The main focusing lens was located on top of the pinnacle of Skyreach itself, whilst the secondary lens was located on top of a nearby, shorter spire. Two other focusing lenses, which Reshad called false suns, were located at the Windswept Terrace and Terrace of Dawn. It does not seem to be as powerful as the original Breath of Rukhmar, based on the original's description in Chronicle.
During the rise of the Old Horde, Kargath Bladefist temporarily allied with the Arakkoa Outcasts to kill the Adherents of Rukhmar and destroy the weapon. What the Outcasts lacked in numbers they made up for with their magic, and destroyed the mechanism atop the city. A blinding explosion tore through the sky and set the heavens aflame.
Warlords of Draenor
This section concerns content exclusive to Warlords of Draenor.
At the beginning of the Spires of Arak storyline in the alternate Draenor, the weapon fired on Veil Akraz, burning it to the ground. Veil Zekk was also destroyed. The weapon also played a role in the fight with High Sage Viryx, as she called down the powerful focusing lens to scorch the ground beneath her foes. In this universe, it was eventually rendered useless by destroying the two smaller focusing lenses.
A fel-empowered version may be under construction/reclamation at the Farlight Terrace, if not just a similarly-shaped object.