I have two quests. Both located in the circle where I'm standing. The issue is, I don't know what it is. I scanned all the monsters using my quest locater, non of them had the Quest symbol on them. So then I opened the chests around it (about a dozen) and still nothing. This quest rewards with an eight capsule holder, if that helps. I got it from the hippo that's not too far down the road from the castle town.
You can click on the thumbnail to make it bigger.

Mouse over the circle to get the name of the monster/thing you need to kill/pick up and just look for w/e matches that name. Make sure you're tracking the quest too.

Theres a dungeon there. walk to the end, and into the portal

There is no dungeon or portal anywhere to be seen where I am.

-Edit-
Well, that was odd. Not sure what I did but apparently I completed it.