Adora had just defeated Horde prime and saved Etheria from his tyranny. She rejoined her friends, and the one she loved. The heart of Etheria had dispersed its magic from the core of the planet, bringing a new age of magic to the universe. It was after She-ra/Adora slew Horde Prime and released the Heart of Etheria to reverse the planet's destruction, did things begin to turn dark. In a secret vault in that kingdom of Mystacor, a big red box the size of a large stone stood on a pedestal in the darkness. It began to react by shaking, resulting in a crack that revealed a small light coming from inside.
In the kingdom of Salineas, another vault deep in a cavern of water held a box similar to the one in mystacor, however it was white. It too shook, but only for a second. It then stopped and rested still.
In the Fright Zone, a third box sat in hordak's lab. Unlike the last two boxes, it was metallic and moved like 3D pixels. But it stopped moving and rested where it stood.