Mysterious Hidden is the part of Mysterious Swamp concealed behind an extra veil of fog.
Majorist's festival dialogue points the reward toward an Artifact hidden beneath the swamp fog, and the plants later notice that the swamp center seems to hold more fog than it should.
Applayer once sensed a musical rhythm at the center of Mysterious Swamp, tying music, fog, the Artifact, and old mechanisms to the same place.
It keeps the swamp's eerie mood, but behaves more like a fog-locked chamber: the entrance is revealed by clues, return visits, and puzzle logic rather than by an obvious road.
For that reason, Mysterious Hidden is not just an extra fight. It is the concentrated answer to the question of how much the swamp is still hiding.