Talking to Herman Toothrot is key.

If you've met Herman Toothrot three times without brushing him off, he tells you he lives in a fort.

At the edge of a volcano.

Have you seen any volcanoes?

Or any volcanic looking craters?

From the map screen that includes the south shore, go west to the next map screen.

Herman Toothrot's home, the fort, is at the top edge of the crater.