The grotto is a large labyrinth where secret experiments are performed by the castle wizard Fathis Aren and his conjurer allies and this make the area both uncomfortable and very dangerous.
Unless the Hero managed to locate the secret entrance in the middle of the Niben Bay they will arrive from Castle Bravil through door and will immediately be faced with the unusual sight of a Dremora guarding the first intersection.
The gate to the left leads to a barebones underground maze. Both tunnels ahead leads to the same chamber, with knee-deep water and several paths to choose from. If here for the quest, take the southern or western path and skip the next section. Otherwise, locate the old wooden door and pick the lock.
The door reveals an entire subsection inhabited by conjurers. The dead end to the north contains several piles of bones and a common soul gem. A conjurer will most likely detect and attack so be prepared.
To the south a wooden door says ‘key required’, although no key exists. The northern chamber contains a minor loot chest and a poisoned apple, while the chamber opposite the locked door gives access to the rest of the conjurer lair, a large room filled to the brink with treasure, potions and rare ingredients, along with alchemy equipment and two bedrolls. Backtrack all the way to the watery room south of the wooden door then head east until the knee-deep water turns into a very deep, flooded pit.
The pit is inhabited by a few slaughterfish and is incredibly deep. On the way down, look at the map and note the underwater path leading north; this is the path circumventing the locked door and the only way to proceed in the dungeon. Ignore it for now unless doing the quest, and dive even deeper into the pit until one see a hole at the bottom.
This is the lair of the unique Giant Slaughterfish, so prepare for an underwater boss battle. Make sure to loot the dead fish for a huge amount of valuables then swim west towards a tunnel where the Hero will find door out, the secret entrance from Niben Bay.
Backtrack to the tunnel leading north and swim until they finally surface near an alcove with some lower level equipment. At the next intersection, ignore the left path as it leads to the other side of door and head right.
As the Hero approach the last intersection, they will most likely be detected by the boss-leveled Dremora, so dispose of him and consider picking the lock of the nearby door, even if it will only net another fight against a leveled Daedra, a restoration chest and some alchemy equipment. Finally, pick the Nirnroot and go through to the locked door, the entrance to Bravil Wizard's Lair.