Hythloth is the only way to the Realm of the Gargoyles, thus you have to enter it. The dungeon is infested with Daemons and Dragons, making it the hardest in the game. Strong spells and the Storm Cloak are a must to survive this horror. Also, you have to walk through lava, which isn't healthy as well.
On the first level, you must reach the ladder 1 in the north-western part of the level. On level two, the ladder in the lava in the middle of the big hall looks obvious, but is fake. Instead take the southern ladder at 2. On level three follow the long corridor and then go the eastern corridor south to reach the ladder 3 to level four.
Reach the south-western part of level four to find Captain Johne at 4. Talk with him, learn the language of the Gargoyles, then descend the ladder at 5 to enter the Realm of the Gargoyles.