Disclaimer! I am far from an expert in lore (but i guess blizz has strayed away from the lore a few times anyways so 
), I also realize that there are some gaps in the story. Nonetheless, could this be something?
Zovaal is an Eternal One that through time has become affiliated to the void lords, he has tried to conquer or destroy the slumbering world soul azeroth for an eternity as it is viewed as the single hanededly biggest threat to the void lords if she would ever awaken. Sargeras shared the same view about azeroth (more on that later). The first attempt by the void lords was to sending the 4 old gods N’zoth, Yogg sargon, C’thun and Y’shaarj to azeroth.
Other attempts includes Zovaal using the Nathrezim. When Sargeras found the Nathrezim on a void consumed planet he thought he had them by his side, but the Nathrezim who are masters of manipulation had already sided with Zovaal. Zovaal used the Nathrezim and Mal’ganis to trick Sargeras into believing they had a partnership, and manipulated the Burning Legion into creating the Lich King. The main purpose of the Lich King to weaken azeroth before the Burning Legions second invasion.
Zereth Mortis turns out to be Azeroths ancient name that has been lost in translation through the ages.
With Sargeras imprissoned and his sword plunged into Azeroth, Zovaal now sees an oportunity to finish the job himself. . Zovaal has through Sylvanas opened up the veil between the realm of the living and the dead in Icecrown and It will be here that Zovaal makes his entrance into our world of the living. Upon following Zovaal the champions of Azeroth realize that their power of light is strongly hemmed by the saronite fortress of Icecrown, which forces Jaina to teleport everyone away. With the Champions out of the way Zovaal makes his way to Silithus to turn Sargeras blade into a Mourneblade that can finally complete his eternal mission of consuming the world soul of Azeroth and thus undoing our reality.
In the end, when all looks lost, Azeroth wakes up, being severely wounded after years of turmoil from the Legion invasions, Deathwing breaking the world, Sargeras’ stabbing and now Zovaals attempts she turns out to be a true force and manages to overthrow Zovaal. Next expansion will be all about healing her and our world back to what it once was.