Enter Darkwater Pass and turn left. There is a chest to the right near the entrance as well as a burial urn. Follow the path until coming to a waterlogged clearing. Here you will face off against some leveled Falmer foes. Instead of heading straight on, go to the left. You will encounter Chaurus here. Go up the stairs until you reach a room with two doors and a trapdoor in the middle. You encounter more Falmer. Make sure to loot the Falmer Warmonger as he possesses a "shaman's key". In the room to the left, there will be an arcane enchanter along with the enchanting skill book Catalogue of Weapon Enchantments. This book is hard to see as the color scheme ties in with the enchanter. To the right of the entrance is a discrete lever that is extremely hard to see. Pull it and a stone wall opens up. Follow it down until you see Derkeethus. A conversation prompt will start and he asks you to rescue him. You can either pick the expert level lock or search the burial urn just inside his cell for the key. Escort him him out of the cave and back to Darkwater Crossing. He will then tell you that he wishes to follow you on journeys in the future.