Six years ago, when the Cross-Eyes targeted and killed the founder members of the En Family for over two years, Tenjin, one of the remaining members discovered the hideout of The Boss and quickly informed it to En, who sent Shin and Noi to finally kill him for good. They were ambushed by The Boss who used Tenjin's severe head as a bait to lure them into a corner and easily defeat them using his powers to weak them. En knew something was off about that and he personally went to the city and saved his cleaners in the last moment. Using all his power to overcome the fear The Cross-Eyes Boss had over him, En engulfed the whole city with his magic, becoming a morphed wasteland filled with mushrooms, the inhabitants could no longer live in the surface and recreated an underground city using the subways, the mushrooms still grown, slowly covering even the underground.
After Kaiman rescued and snapped out Nikaido from En's control, Asu used a magic door to sent them as far as he could from En's Mansion so the contract inside Nikaido would not regenerate. They ended in the middle of the wrecked and abandoned city, looking for a place to hide from the Devils, who would tell their location to Chidaruma, and subsecuently, En.
The duo found the underground shopping center/remains of the city inside the subway, as is explained later, once the city became a toxic wasteland the survivors build what is now the underground city of Mastema, where people unable to use magic live as there's no other place for them to live.
They stumbled with Natsuki, a resident of the underground city and a newly enrolled Cross-Eyes, looking at Kaiman's own Cross-Eyes, she immediately took him as her superior and help them to get new clothes, food and shelter.
After Natsuki, with the help of Kaiman, got rid of her abusive boss who tried to kill her after she opposed to the idea to sell fake Black Powder, (believing that was betray everything she believed about the Cross-Eyes), decided to accompain them to the Cross-Eyes hideout in Berith, leaving the city without paying for anything they got.
En's death at the hands of The Boss felt even there, his magic was banished and the mushrooms that rendered the city uninhabitable in the surface where gone, rotting from the ceilings and walls of the underground city.