The Protean Dungeon is one of the oldest dungeons on record, and it's sufficiently unlike other dungeons in many aspects that the more narratively-minded people around sometimes speculate that it might be the first, and perhaps even something like other dungeons' "progenitor", if that's something that makes sense. It was not the first dungeon found, because it's in the middle of the Australian Outback so that took a bit, but it's extremely big, extremely powerful, and, most importantly, it changes.
Now, every dungeon does to some extent, of course. Right after it's cleared, every dungeon closes for a bit, and the next time people go in it's different than last time. Sometimes it's minor, a moved hallway, an extra room, slightly different and more powerful monsters, and other times it can even include entirely new levels and bosses. But the Protean Dungeon changes more, much much more, than any other dungeon does, and much more quickly. A regular dungeon whose first level is themed around wetlands like the one Elisabel just visited would very rarely change that theme; the Protean Dungeon's first level has seen such variations as deserts, jungles, ancient temples, ruins, office buildings, labyrinths, pyramids, tombs, graveyards, the surface of the Moon, mazes of impossible geometry, underwater caves, post-apocalyptic cities, volcano lairs, and much more. Its monsters are just as varied, naturally, always keeping in theme with the dungeon itself.
Furthermore, no one has actually ever beat it. A few parties have managed to explore past the first level, and some even past the second, but no one has defeated any iteration of its third level's boss, and no one has much reason to expect the third is the last. Trips into it are proper camping trips, lasting multiple days and requiring a lot of preparation.
And no one understands how.
People's best understanding of dungeons is that they need to spend mana, or at least something like mana, just as much as humans do. They must have access to different kinds of magic and maybe different resources, seeing as there is no known spell to create monsters or pocket dimensions, but it does seem like there is some cost to changing, and some way the dungeons have of acquiring or regenerating those resources. Statistically, dungeons that change their layout too much (such as by changing the theme of a level) tend to change into a less powerful alternative before powering up again; new floors are almost always easier and smaller than existing ones when they first appear; and the biggest changes occur whenever people die inside, suggesting that they "consume" those they kill, somehow. By all accounts, the Protean Dungeon is breaking the scales set by every other dungeon on record.