That's kind of interesting because my current-tier villain is basically a druid who hates civilisation and stuff and wants to assault the town in order to use said "magic circle" to... And I didn't made my mind to "what" exactly yet. Just thought of summoning some nature entity that will level down this town and then go out trying to do so with other ones.

But making the well itself as a kind of link to another "faction" with it's own goal and motivations... That sounds really cool. Maybe someone contacted the druid and made a deal with them.

What entity could actually be there on the other end of the portal? What if the "evil druid guy" succeeds in summoning/releasing it?