Sousei no Onmyouji - Twin Star Exorcists Wikia

Magano as seen in the anime.

Magano as seen in the manga.

Magano (禍野 (まがの)? "Field of Misfortune") is an alternate world with the appearance of a barren land and ruins of buildings that takes people into another dimension. The negative emotions of humans gather in Magano creating impurities, who can travel to the human world through gaps in the limit between the two.

Exorcists are able to travel to and leave Magano with a special talisman. In the manga, only powerful exorcists are allowed to do so, as it is a very dangerous place full of Impurities. If weaker exorcists were to be killed in Magano, their powers could be absorbed by impurities, strengthening them.

Magano was once the parallel world to the real world. It had been filled with human beings, greenery, and civilization. The Magano seen now was made by Abe no Seimei.

True Levels

Chap. 35, Shimon explained about the depth of Magano on Island

The True Level Magano (真層・禍野 Shinsou Magano?) is the Magano located at Tsuchimikado Island. This Magano is much more dangerous than those outside the Island, and consists many levels and branches. The Shinzou Magano started at level 2013 and become deeper when the number decreasing.[1]