The Demon Shaft is an optional dungeon that was added as a bonus to the US and European releases of the game, and which only unlocks after the Dark Genie is defeated. As you'd expect, it's also the longest dungeon by far (comprising 100 floors) and has quite a few dangerous enemies. Most of them recycled from earlier dungeons, but now with higher stats and resistance to all but one or two elements (or in the case of White Fangs and Hearts, immunity to all elements, requiring you to disable your weapon's elemental properties to harm them at all).
On the plus side, the treasure is pretty good. I found two more Five Foot Nails (which I Synthed and gave to Ruby and Osmond) and the dungeon seems to be pretty generous with back floor keys, which gave me a steady supply of gems and salable goodies (which I converted to enough gems to max out Xiao, Ruby and Osmond's weapons). There are also none of the pointless character-specific obstacles in the way, so you can just stick with Ruby, Xiao or Osmond and clear out floors in pretty short order.
Other than that, there's really not a lot to say about the Demon Shaft - it's not very exciting to watch because of its sheer length, and the repetition isn't even broken up by a new Georama challenge. So I went ahead to the last floor off-camera just to show off the one original element of this dungeon, the game's optional boss. His name is Black Knight Pendragon and he... honestly kind of sucks.
Your reward for beating him is the Chronicle 2 sword, which is far and away the most powerful weapon in the game statwise. It is a bit pointless since there's nothing left in the game to use it against, but I suppose it's more of a trophy than anything.