Head for the door that you attempted to pass through earlier. Head down the slope in front of you to where you can see the water below. This will lead you to the Gorgon's Labyrinth. You will see Taken Hobgoblins as you make your descent. Take them out from cover and carefully. They are all set up to snipe at any Guardian they see entering the area. Take them out and then drop over the ledge.
Use your Jump abilities close to the ground to minimize the landing damage. When you get to the bottom you will need to deal with a number more Taken Hobgoblins. Like the first one above, they are set up to snipe any intruders. Destroy them one at a time to avoid getting swarmed by the darkness orbs that defend them and attack their killer upon death. There are at least four in this area.
There is one nearby where you made the descent, on the top of the slanted rock nearby. The next two are along the wall on the left-hand side. Now head forward, following the stream. Your target is just around the next corner.
You are looking for this large Taken Hydra. Be careful because there is another Taken Hobgoblin opposite the hydra that will start attacking you.