The Light, or The Other Side is referred to as the other world of Alters and where the power of Alter originated, a place bathed in psychedelic light, consisting of mountainous terrain and very little oxygen. One can achieve the light when encountering "the Crystal."
The Light/Other Side was first experienced during a disaster that occurred in Japan twenty years ago called the Great Uprising, which devastated part of the archipelago and ripped apart a large of land and raised it higher into the atmosphere. This portion of land became known as the Lost Ground (island) and seen as the birthplace of Alter Users, a small generation of humans and animals that make up two percent of the global population, but as the years progress, that number has increased.
During a battle between Kazuma Torisuna and Ryūhō Ryu, a second uprising occurred and drew the attention of the mainland people, desiring to exploit the power of the Other Side for economic gain. To do this, they send their refined Alter User, Kyōji Mujō, to the Lost Ground, but as he obtains greater power after absorbing The Crystal, he develops delusions of grandeur and defects, wanting the power of the Other Side for himself, but he is soon defeated by Kazuma and Ryuho and sent to the Other Side.
As a final attempt to gain control of the Lost Ground, the mainland government sends their army of refined and artificial Alter Users, but they are defeated by Kazuma and Ryuho, and as the series comes to a conclusion, the power of the Other Side is still something to be desired by the mainland government.