Katherine did not particularly like Mardum, even though she herself had been born there.
As the queen of the Nerubians... well, something needed explaining here. Mardum's Nerubians were not the Nerubru Nerubians barely surviving underground in Northrend on Azeroth. Whether in strength or level of existence, the Mardum Nerubians surpassed Azeroth's Nerubians by more than a single tier.
They were a demonic species, supposedly among the twisted creatures the Titans had once imprisoned on Mardum. They were exceptionally adept at spatial magic, and the strongest among them could even tear through spatial barriers with ease. Moreover, they were brutal by nature. Under their queen's leadership, they were like a migrating swarm of locusts, undoubtedly threatening the balance of the universe.
However, it was said that after Sargeras cleaved the planet Mardum apart with a single stroke, one of the Nerubian queens had intervened to save it from destruction. These were all rumors circulating among demons, but they did not seem entirely unfounded. The Nerubians had unquestionably joined the Burning Legion, yet their current queen, Kethara Nightblade, had remained on Mardum all along, seemingly observing some ancient precept.
In truth, Katherine was indeed guarding something of vital importance, but it was not an ancient precept. It was... the Keystone!
The Pantheon had fallen hundreds of thousands of years ago, and the Burning Legion had been founded even earlier. When the Dark Titan resolved to launch The Burning Crusade, the Keystone had been left on Mardum. That divine object's existence spanned longer than many planets had existed, and Katherine was merely one of its countless successive watchers.
But a duty was a duty. As long as that sharp, dark green stone suspended at the very center of the Fel Hammer did not leave Mardum, Katherine could not leave this world within her