Darkstorm

The Darkstorm was a terrible, magical force that could unmake and destroy Azeroth and every living soul on it. It feeds on life.

An unnamed naaru was foretold to stop the Darkstorm, and it lent its grace to shield the world. Prior to the Third War on Azeroth, the naaru was protected by the Order of the Seven Suns led by Greydon Thorne, with his brother Silverlaine becoming a member as well.

Highlord Xaraax of the Burning Legion wished to spread the strength and corruption of the Legion across the universe, with Azeroth resisting due to the protection from the naaru. Greydon was bestowed the Diamond Blade who kept it for himself instead of giving it to the superior swordsman that was Silverlaine. The two brothers fell out, and Silverlaine stole the naaru's blade and went to confront the nathrezim. Instead of killing him, Silverlaine was convinced to join the demon instead. When Silverlaine returned to the order, he killed the naaru, shattering the Diamond Blade and the order. Despite the Legion's victory, the Darkstorm couldn't come. The naaru's essence survived in the shards of the Diamond Blade. Over the years, Greydon and later his son Aramar and adoptive daughter Makasa Flintwill were gathering the shards all over the world.

Some time after the Cataclysm on Azeroth, the Darkstorm was kept in an altar in the Hidden citadel in Hellfire Peninsula as a purple mass of liquid shadows, ever moving, ever whispering of unspeakable secrets. In the meantime, Greydon and Aram were captured in the stronghold, and Makasa, wielding the reforged Diamond Blade, led an army there in order to rescue her family. If Xaraax could obtain the Diamond Blade, he could destroy it and nothing would stop the Darkstorm. A great battle ensued that resulted in Malus, who had been deemed useless by the demonlord, betraying Xaraax when he was about to corrupt the Diamond Blade. Malus impaled the dreadlord on the altar that swallowed him, hoping it would send him to the Twisting Nether. Now, the fel-corrupted Malus ordered his servants to bring his bodyguards, the Gordunni ogres as well as imps to fuel the Darkstorm, and then he would feed it Greydon, whose essence it needed. The ogres and imps were sucked and the slithering mass became a portal, the beginnings of the Darkstorm, and with whispers emanating from within, louder more sinister, and in language too frenzied to make out. Soon it expanded into a black vortex. In the following chaos, Makasa and her friends, Malus and his demons, and the Hidden fought against their enemies. Eventually, Aram gave Makasa an opening to deal with Malus. As Makasa killed Malus with her Diamond Harpoon, the Darkstorm roared, and then its energies dissipated, ending the Darkstorm threat.

Speculation

 * Due to its description as a purple mass of liquid shadows, with whispers in a frenzied language, it is likely the Void.