The Lunar Lenses before recharging
The Lunarium is a central location within Mystacor, it houses the Lunar Lenses. It is a dome shaped room covered with ancient crystals, and has an oculus in the center of the ceiling. Every month, Mystacor's three moons align in an eclipse that can be seen through the ocular window. The the lead sorcerer uses the Lunar Lenses to capture the moon's magic to recharge Mystacor's magical protection barrier.
It was inside the Lunarium that Light Spinner and Micah cast the Spell of Obtainment, which either killed or banished several citizens of Mystacor to an unknown realm. In the episode In the Shadows of Mystacor, She-Ra, following what appears to be Catra's voice, runs into the Lunarium. The voice changes to Hordak's and in a frenzy, She-Ra accidentally destroys several thousand year old crystals with ancient writings as revealed by an exasperated Castaspella. Later on, this building is also where a Shadow spy sent by Shadow Weaver attacks She-Ra, Castaspella, Glimmer and Bow.
During the recharging ceremony