Omegahugger
Inattentive Idiot
A nice rule of thumb is that a Necromancer with the Liber Mortis is a threat that takes the entirety of the Empire to reliably put down.Problem with Naggaroth is that Mathilde's sanity and life may well not be enough for that place.
They are a magical superpower that even a LM necromancer wouldn't be able to beat, they probably know the second secret of Dhar, and they can be just as competent and efficient as the chaos Dawi when they want to (ie not right away like the chaos Dawi, but if they survive the alpha strike, which they'll probably will).
They are a real hard target. Even Necrothilde can't take them... at least, not unless she levels up quite a bit more than current Mathilde plus necromancy.
Now, could the Empire, with all its might and grandeur, take on Naggarond? The answer is no, no it cannot. And unlike when I usually make that sort of statement, it is not meant as a diss against the Imperials.