Tropical Cave - Water's Edge to Hidden Shores
Go through this cave like any other dungeon. The trident-wielding troglodytes only have melee attacks and aren't anything you can't handle. Also search every corner for chests.
At one end of this cave, you'll enter a large room where some beaten-up prisoner tells you to look out. Several slavers carrying large, dull swords will ambush you. A single slaver is easy to beat, but if several of them surround you at once, they can quickly ass-rape you.
After slaughtering all the slavers, the prisoner will introduce himself as Rondo Blackwright, a shopkeeper. He doesn't know about the Horn of Ill Omen anymore than you do, so you should find his brother, Lazlo Blackwright, for more information. By freeing Rondo, you also have a shopkeeper to sell your unused items to. His store is located near the Save Stone back at Water's Edge.
In the same room with the slaver ambush, there's a broken Portal Stone. Unlike the regular ones you've been using so far, this portal will automatically warp you to a predetermined area as soon as you near it.