As the Undead Scourge was cleansed and the lands of Azeroth recovered from the devastation lain upon them cooperation began to waiver among the two dominant races. In the outlying territories where resources were thin, and the peace was not so easily enforced, conflict soon broke out. As the fighting escalated a new and frightening enemy crawled from the ruins of the past to settle the future.
Following the battle at the World Tree and defeat of the Scourge much of the conquered Undead lands were reclaimed for the living. There were numerous abandoned facilities in remote locations that were never found by Alliance forces. With most of the living focusing on hunting remnants of the Scourge or rebuilding there was little interest in searching the wilds for empty structures. Upon Illidan's death and the seemingly benign reign of Arthas in the far North a tenuous peace began between the Orc and Human forces.
Years of strained cooperation passed.
On the outer fringes much of the old tensions remained active in spilled blood. Territory was constantly being fought over. All were oblivious to what was coming.
At the behest of the Burning Legion Undead researchers began looking for sources of energy to sustain the Scourge. In the darkest hours before the final battle there was success, although those responsible knew nothing of their accomplishments as the facility was consumed in an instant. Out of desperation the consequences of their dark magics had been ignored. A slight dimensional tear had been opened - to where was not known. As the years passed the rift slowly expanded consuming its tomb of debris until the ground was void of substance. Sinister magics began to flow into Azeroth as the swirling nethers of the Second War had, darker than even Archimonde could have summoned.