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.
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: Details about a quest, mission or other Final Fantasy XI in-game storyline follow.
Riko Kupenreich: You've got yourself a loan. We'll give the Kupeliaure brat what he's asking for...heh heh heh.
Riko Kupenreich: What, you thought you'd be getting' cash? I don't think so. We'll be keepin' this deal on the down-low, ya see? This draft is good for all the gil Toto asked ya for. And you're gonna carry it so that nobody's gonna see even a bulge in your pocket. Got it?
Riko Kupenreich: Anyway, I'll be gettin' my people on this. Uncle Riko'll find out what Toto is up to.
Riko Kupenreich: In the meantime, ya better not speak a word of this to no one, ya hear me?
Riko Kupenreich: If news of Toto's plot went public, nobody would trust us Moogles ever again. That wouldn't be good for business. And what ain't good for Uncle Riko's business, ain't good for no one's business. Ya catch my drift, kid?
Goblin Repossessor: You swear life of unpaid servitude at his new Masquerade Mog House, maybe he work you out deal.
Moogle: (Ooh, Master! I've always wanted to be an actor, kupo!)
Moogle: <Ahem>...I mean, absolutely out of the question! Begone before my master wipes this freshly polished floor with your worthless hides, kupo!
Goblin Repossessor: Har har har! You funny. Anyway, you know deal now, so my job done. You want save girl, you sign contract, bring to Chamber of Oracles. ...Or else! Har har har!
Moogle: Oh Master, whatever shall we do? Is there no end to a moogle's miserable luck?
Moogle: I won't--I can't leave this Mog House behind! But just the thought of my poor Kupiruru, forlorn and forsaken deep within the den of those despicable brutes...
Moogle: It's enough to move a poor moogle to tears... Would that I could flee this flimsy coop and fly to her side this instant! But alas...my contract with the MHMU keeps me pinned here to my post, kupoooooo...