Magnus’s RPG empire on the portal.

These demons are getting on my nerves... they're killing innocent people, we have to kill Biara and overthrow the Overlord to kill the Emperor and end the war...

Well, first of all, you're a demon yourself, if you look at you.
Secondly, since Biara, judging by your story, has separated from the Overlord, we need to wait until they destroy each other.
And then they'll destroy the Emperor. We don't need to interfere, we're not the Dark Brotherhood. What are your actions?