You leave the strange mirror alone. Saucix leads the party to the second to last unexplored room, opening the third door leading from the mink's room.
Behind it is a small room. A huge humanoid is sitting on the floor, leaning back on the wall, immobile. The creature seems to be made entirely out of ice, with frostwhite eyes and icicle teeth. You guess it would be about eight feet tall if it stood up. In its left hand it clutches a white stone orb, identical to the one Saucix found. In its right hand is a long sword, covered in frozen blood. Lying on the floor before the humanoid are two goblin corpses, frozen completely solid.
The creature's torso has a pair of long "scars"; tears in the ice that appear to have been filled with new ice over time.
Remus: Harry... I'm a werewolf.
Harry: Are you fucking serious?
Remus: Well yes, but I don't see how that applies here.