Inside some of the crypts in the black forest, doesn't matter which black forest, there is a glowing red runestone. Or sometimes you can find one on the back side of one of the towers but that's more rare. If you click on that, it will mark the position of the nearest location of the next boss, The Elder. There's more than one Elder location on the map, the runestone only shows you the closest one.

There's someone else in the Black Forest that is much harder to find and might require some random sailing. Finding them is optional, but helpful.

Stay away from other biomes when you're sailing around. Be terrified of brown grassy fields.