The Crystal Dome Generator was a machine used by the Evil Horde to place an indestructible dome over Whispering Woods to keep members of the Great Rebellion from getting out.
Inspector Darkney brought the Generator with him from Horde Prime when he came to inspect Hordak's progress against the Rebels. The plan failed when the Dome was lifted up by the combined might of He-Man, She-Ra and Swift Wind, after which the Generator was picked up and destroyed by He-Man, the most powerful man in the universe.
Darkney standing next to the Generator