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Then it must be bug: I lose the key every time I go into the dark room and exit on the other side, even if I don't fall off. For this reason, I haven't been able to get to the final boss for the second time. Do you think resetting the game can help? Also, is it intended, that the enemies go down in two hits after the first boss "battle", instead of in three they took initially?
Alright, I've finished the game, and now I get how it adheres to the theme (no spoilers). :-) I also think I've figured out what caused my key to disappear: after defeating the boss, another player character spawns on the opposite side of the map for some reason, and it also responds to the controls, so when I move into the dark room, it can easily fall into the pit and lose "my" key. I worked around it by walking over him, so both "my" PC and the clone are in the same cell at all times, but the game eventually hung up when I tried to enter the dark room again. I was able to beat the game after the restart.
Also, not sure if it's a bug or a feature: the attack option during the final boss fight reads only "Let", and in the final log, I can only see what appears to be parts of the text. Is it supposed to be like this, or is it just the display elements eating up large fonts?