Once you zone into Sea Serpent Grotto go to (C-8) which will bring you to an area hidden on the map. You will pass through two Cracked Walls leading to a hidden room. In the very back of the room you will find the 'Shady Sconce'. Check it for a small cut-scene. (Make sure you check it twice to get CS with moogle.)
Then head to (J-11) and go through the Ornamented Door.
A Sahagin Key is required to open the Ornamented Door. This is a one-way door.
Make sure to use some form of Sneak when you go past the Ornamented Door, as monsters will aggro at lvl99.
You do not need to go through the Cracked Wall in this area, so be careful not to run too far! If you do, you may need to go around again and get another key!
It's recommended that you bring some form of Warp as there are True Sound NMs on the way out from the area behind the Ornamented Door.
Oh, woe! Having exhausted all other options, your star-crossed moogle associate is forced to seek aid from the shadiest of scoundrels. You have an appointment with the underworld moneylender Riko Kupenreich at his office in Sea Serpent Grotto.
SPOILER WARNING: Click here for additional details about this story line.