A summary of the next ~6 paragraphs: The Nightmare of Druaga is
fucking ridiculous, in a way that somehow appeals to me.
Nightmare of Druaga's post-game dungeon is like an approximation of a "real" Fushigi no Dungeon game, in that it has only randomly generated floors (except for a boss every 20th floor), there's no checkpoints to continue from, and the monsters' exp. levels scale with yours, so grinding doesn't help (and eventually you reach level 99 anyway, whether or not you're trying to get there).
However, unlike the average roguelike, most of the items you find along the way (i.e., anything that isn't a potion) aren't going to be of any use until you get back to town, where you can sell or combine them with other items. So it's actually all about what you're bringing with you into the dungeon, which amounts to grinding to improve your equipment.
This is where the game, in a way, most resembles the original Tower of Druaga, which was full of hidden items, one per floor, several of which were required to finish the game, and all of which had to be uncovered by doing ridiculous things which are only occasionally even hinted at in the game. NoD has these too, two per floor (one silver, one gold, each has its own conditions to be met before it appears; also, you can't even make a gold chest appear until you obtained that floor's silver chest, and then come back later) in the main dungeons, but you don't technically need any of them, they're just helpful. But for the post-game, it isn't enough to combine just any items. You need equipment with good stats to begin with, and you need good items to combine them with too. You also need to find equipment with the right abilities. Especially for your weapon, since it can only have 5 abilities, and you can't remove the ones you don't want, you need a weapon that's powerful and doesn't have any useless abilities to begin with. You don't find these randomly, not ever.
From each floor in the main dungeons (except for boss/story floors; there's about 115 between the four dungeons) you can enter a different 4-floor "bonus dungeon." These dungeons are randomly generated, to an extent, and are populated by the same enemy types that were present in the floor you entered from, except now they're scaled to your exp. level, and don't give any exp. points. They do, however, drop items frequently, and each bonus dungeons has a specific set of (non-rare) items that can be found there. The 4th floor of each bonus dungeon is still randomly generated, but it also contains a giant boss monster (a japanese site for the game calls them 'gatekeepers'). These things are extremely powerful; having even one regular enemy approach while you're fighting one of them puts you at great risk. If you can defeat them, you can get a rare weapon or piece of armor for your trouble. There are two important points to be made here:
1. Each of these ~115 bonus dungeon bosses has its own, "unique" item drop. It is possible to get multiple of one of these items, but only by going back to that same boss and killing it again (many of the gold/silver chest items are also "unique" in this way).
2. The boss isn't guaranteed to drop anything. Or, it might drop a gem, which are relatively unimportant. You might have to kill these things 4+ times to get the item you want from them.
Here's where it gets
. These giant boss monsters
also appear on certain main dungeon floors. But, like the gold/silver treasures, they have some absurd requirement to make them appear. The difference there is, at least the treasures are on every floor. There's no way to know which floors they're on, OR how to make them show up (except reading about it on the internet). There's one on 4F of the first dungeon, which makes it easily accessible. This one has a second, even rarer weapon drop, the Tempest Bringer. This is my current weapon.
So anyway, this game is totally awesome and everyone should play it.