Not precisely a scenario per se, you're supposed to visit the Sealed Cave and Sealed Labyrinth to level up and gather equipment between scenarios proper. However, there is another point to coming here, so it deserves a section in the walkthrough. In particular, you want to have done everything here before you begin All Things.
The seal at the door is initially non-functional. As you complete certain scenarios, the crystals will be restored, and after Temple of the Wizard it should be operational again, which will prevent you from entering the cave. You can only get in after that if you have the Golden Heart from Lefia, which will open the door.
On the maps, you'll note that there's only one save point, which is right at the beginning, so don't go in over your head. As you level up, you'll stop meeting enemies in the lower levels, so trekking back here to save is more inconvenient than dangerous. The Restore Lute skill is invaluable when exploring this dungeon; it's a lot cheaper than carrying vast quantities of potions, and you want to save your MP for use in combat.
The contents of treasure chests appears to be somewhat random. According to some people, the more powerful items only appear after the seal has been restored, but this hasn't been verified. However, it's possible to change your luck with the good old "reset and try again if you get the wrong item" trick. Note that the random seed is saved, so if you simply reset the game and try again you'll get the same item every time. The trick here is the "shuffle luck" command (fourth option on the bottom tab of the main menu), which appears to reseed the random number generator. If you push that a few times each time you reload, eventually you should get a different item. Each chest may have two options, depending on the random seed.
There's one point in particular where the maps may prove difficult to follow. There are two paths between the third and fourth floors of the Labyrinth, and the fourth floor is divided into two areas which do not connect. What you need to to is to head to the fourth floor, follow the path to "Switch 2," and activate it. This opens the door to "Switch 1" on the third floor, so go and activate that; then return to "Switch 2" and turn it off again. Now you can pass through the other keyhole-less door on the third floor and reach the other areas of the fourth floor, which will take you through to the deeper levels.
Follow the maps until you reach the final room of the deepest level (marked with
@ on the map). This is the throne room of Amugnon Castle, and King Daruk's corpse is still here. Examine it. At this point you may be privileged to fight Daruk Golem; beat it to unlock the best ending.
If you have the Golden Heart, you'll now "receive King Daruk's heart," and then you'll be teleported to the entrance, and the door will be locked again permanently.