The news of Mardum's destruction could not be hidden from the Burning Legion. Although most of that world had already shattered, special circumstances meant that the Legion still used it as one of its bridgeheads for invading other worlds. In fact, finding a planet like Mardum in the Twisting Nether was extremely difficult.
That Demon Star had been shattered by a single stroke of Sargeras's blade. Later, through an extraordinarily coincidental transformation, it had become a shattered world with an almost nonexistent spatial barrier, yet an intact continent. This made opening the Legion's portals exceptionally convenient, which was why the demons valued it so highly.
In truth, Mardum's situation was very similar to Draenor's. But when it came to that almost nonexistent spatial barrier, even in the shattered world of Draenor, only Netherstorm barely qualified.
In other regions, such as Shadowmoon Valley, the Bone Wastes, and the Blade's Edge Mountains, the spatial barriers still more or less functioned. After all, Draenor had exploded from within because Ner'zhul had drawn in too much void energy, almost the complete opposite of Mardum's destruction entirely from the outside.
Kil'jaeden was furious at Illidan's betrayal. The direct consequence of that fury was that as many as 200,000 demons took part in this invasion of Draenor. Although it fell far short of the offensive that had invaded Azeroth, Draenor was only a shattered world, after all. Committing so many demons at once was already sufficient proof of the Deceiver's resolve.
He would capture Illidan! He would punish him for his audacity!
Most importantly, he would reclaim the Sargerite Keystone. Having that thing stolen was simply too fatal a blow to the Legion's order.
And precisely because of Kil'jaeden's resolve, three of the four portals opened by the invading demons were in Shadowmoon Valley. The demons