And, like Hellmouth in this section said about Sheo, I say Dagon is the fighting equivalent, in terms of revolts. Every time he comes to take over, people revolt. If the mortals cannot stop him, I think he would "fully" continue on and "enslave" or destroy everything until a point came where we was stopped. In the end, I think he wants to simply test the strength. Lame, I know, but it makes sense to me, so I'll stick to it. I think many people assume he simply wants to completely eradicate everything, but like other say, I think he's just having fun.
As for taking over... It would end up being due to the weaknesses of "humanity" who cannot rebuke/repulse the advances of the Princes.