The Abyss is basically, a Labyrinth within a Labyrinth.  You actually find out about it when you meet Gotz for the first time on B1.  He's playing with those labyrinth stones and tells you that he's planning to build just that.  But, this place isn't actually accessible until after you've been the game.  When you beat it, the game will ask you if you want to save it or not.  When you do, you'll notice you're character marker on the game save screen turn yellow, to show that you've beaten it already.  Now, just go back to where Gotz's room is and the boulder is now gone and you can go down.

They have a new set of monsters that only appear in the Abyss.  They are VERY strong, enough to easily wipe you out if you're not careful.  They also have random bunchings of monsters (4 Bogey Cats & 1 White Dragon, or a Vampire Lord & Gold Dragon).  Not only that, but the floors are very large (really.. g*d d*mn), and there are many, MANY levels in the Abyss (I have yet to reach the bottom).  Random items can also be found here, and the rarity of those items is also randomized (On one floor, I've found a Cloak, 2 Bishop Orbs, a Megadeth Stone, a Kanesada, even another Queen's Guard Sword).  Other special items, such as Vellums, can also be found here as well as Mutated Stones.  You may also come across other rare items that monsters will drop or items you've never gotten before (such as the Muramasa, a REALLY nice katana), and even find the rare monster materials, such as Incubus Wing.

Another thing, is that occassionally you will bump into a zombie selling medicine, pretty much selling exactly what is sold in Smiley's Shop.  You can also meet Eric the Monster Shop owner in here, and sell/buy materials.  There is also a Rotting Corpse running a Corpsa-Lodge in the Abyss.  You can pay 3000G to sleep and recover your HP and MP, but can't stay the night so you can't level up.  These can each be hit once per floor.

On the 10th level, and other floors in multiples of 10, you will come across a room.  Inside you'll see a small corridor lined with candles, small rectangles covered in japanese symbols hanging from the ceiling (which I later found out were sutra seals), symbols on the walls behind the candles, which all lead to a small shrine-like alter with a floating blue flame on it.  That thing, is the demon creature, Nine Tail (what is it with demon foxes having 9 tails?  Does it mean anything?  What's up with that?).  This is a very, very, very bad thing. Don't believe me?  Go up to it.. engage it in battle, you'll be forced to anyway.. and watch yourself die within the first 5 seconds and seeing that oh, so lovely, Game Over screen.  Yeah... told ya, didn't I?  Bad bad bad.  This monster is, by far, the strongest monster in the game.. even stronger than Bu'Shin.  Physical attacks hurt like hell, magic attacks are strong as hell, breath attacks are also very strong (if you DON'T do Deploy, a single breath attack CAN kill you, trust me).  You may luck out and have his Breath attack only do minor damage, but you really don't want to take that chance.  So, you've got strong physical, magical and breath attacks that are dangerous to you.. but you can only do AA's to protect from 2 of them: either 1.)Deploy & Spell Cancel or 2.)Deploy & Restrict Shot which will leave you almost no chance to use Double Slash, Warp Attack, etc and then still be hit by whatever you don't have protection from.  You can risk it and choose to do one of those, but that's you.  My advice is: get many levels up (at least 50 or so), upgrade everyone's equipment as best you can, healing spells is a must (Will, Fealds, Lafeal, Cathedral), attack/agility/evade up spells is a must (Jaiba,Jiscort,Conairu).. and pray that you don't get your @$$ handed to you within the first round.

Well.. good luck!