Strangely enough, today I hit level 30 and found a new overlord three tiles away from...another settlement!
I love procedurally generated games. They're so...random.
ETA: Commiesalami, that makes a lot of sense in my case. The world has been revealing itself in a circular pattern. I can probably expect to find them more frequently in later levels, then. Rampaging monsters are going to become more and more of a problem, aren't they?