Cimmerian is one of the mirrors in the Hall of Illumination in the Silver Tower at Foxfire Academy. It is directly across from the Lodestar Mirror.
It has the ability to multiply shadows, which can be manipulated by Shades, and it is also one of the tougher mirrors to decipher as many are distracted by their altered appearance.
It also serves as a secret entrance to a Neverseen room with the Lodestar symbol, which formerly allowed you to leap to any of the Neverseen's sixteen former hideouts. The Neverseen used this to kidnap Wylie.
When Tam leaped away using shadows with Sophie and Fitz, Dex was trapped inside. Sandor had to smash the glass, destroying the mirror, in order to free him. It is unknown if it was repaired.