I suspected another deadly spell, that would make my body cramp, and stop my movement, but instead, something else happened. The boss started humming a melody, and from the fog emerged skeleton soldiers I fought at the beginning of my exploration.
They didn't even properly climb their way from wherever they came before they started swinging at me. I had to maneuver around, trying to make my way to the boss, but then the fog thickened in front of me, formulating a wall, and a giant black knight, one of those I fought before entering this chamber, emerged from it, trying to catch me.
That was not the only one, as the chamber got filled with different types of creatures from the tomb. Mages started casting their ice spells, forcing me to deal with them first, as the boss seemed to be too focused on the humming.
