The entrance of the cave is on the west side of the big island of Bergel Island. It’s on the sea level. When you enter, look for the Abyss Nexus, which will allow you to teleport back whenever you want.
After you defeat Rohan, go to the top of the stairs, where he first appears, and pick up the helm he was holding. This is the Witters Plate Helm, which comes with the Rising Torrent Abyss Gear.
The boss fight itself is nothing special, Rohan teleports a lot, but if you use the ever-so-popular strategy of Focused Shot + Mark Target, you will deplete his yellow bar very quickly.
At ~50% of his HP, he will disappear and return with flashes of light. Dodge these until he stops teleporting like mad and continue the fight.
Completing this questline, will allow you to Pioneer Bergel Island. Its Mission Dispatch has a chance to yield Ambergris, but not on every completion.
