In places where it's "resident" like Shadar Logoth, or a direct infusion into a person like with Fain or Rands wound, it is supreme; not even the DO or Creator can overcome it. Away from its centers of power it might be a very different matter. That's especially true if the DOs power is diluted by trying to exert global influence, and thus weaker in any other location than it would be at Shayol Ghul. That's an issue as well; even during the War of the Power the DO wasn't able to bring all or even most of his power to bear on Randland, but the only limit on the evil of Shadar Logoth was that it needed an outsider to carry it from the city. Now that someone has done that the only limits on its power are inherent ones (i.e. that it's not omnipotent. )

If that's the case and Shadar Logoths is weaker than the DO a lot more possible causes exist, and the most likely is that it's simply a kind of "collective unconscious" for the dead residents of the city. Another possibility, since he's the only known survivor, is that Mordeths deathless evil simply consumed them all one by one and then added the strength of their evil to his own.