So.
The start point for this map is the southeast ladder.
Just...look at all those gorram false walls. Look at them, and weep. As a reminder, you need a light effect to see through them.
But that's not all. See how most of the map's got a dark red filter over it? Anti-magic zone. If you're in one of those, you can't use anything that's listed as a spell.
You know what counts as a spell? The only skill in the game that gives you a light effect. Hope you've got enough flashlights to last you the whole fucking floor, sucker!
Oh, and enemies are immune to anti-magic zones. Just thought you ought to know.
And when you finally get to the end of it and reach the central section that's been taunting you for the whole dungeon up to this point? You fall in a pit. With a treasure chest clearly visible off to the north.
So you fall through a whole series of pits that leads all the way back to the first floor, unlock the shortcut, and climb back up once you've resupplied. You reach a boss door, fight the boss (hardly a sure thing since this is an optional area and the bosses in those are deliberately overtuned for when you unlock them), and regain control. Surely, surely the chest you saw earlier will be within your grasp now, right? Especially since both of the previous optional bosses had chests with very nice treasure lying around their lairs, so clearly the established pattern's on your side!
Oh, you sweet summer child.
Turns out there's a locked door between you and the chest, and to get it you need to go farther into the game to be able to unlock the second half of the dungeon.
Just an absolute masterclass in trolling. And if you're familiar with Sting, who developed this game, you know that is absolutely their wheelhouse