demon world; realm of demons
Refers to a supernatural realm inhabited by demons, evil spirits, or dark forces, often in fantasy, mythology, or fiction.
In that game, the hero ventures into the demon world.
魔界から現れた怪物が村を襲った。
A monster that appeared from the demon world attacked the village.
The boy got lost in the forest and, before he knew it, had wandered into the demon world.
That monster is said to have come from the demon world.
He is a resident of the demon world and looks nothing like a human.
魔境 describes a treacherous, demon-infested region often within the ordinary world, while 魔界 is a separate, entirely otherworldly realm.
地獄 is a Buddhist or Christian hell where souls are punished after death, whereas 魔界 is a fantasy or mythological realm of demons not necessarily tied to the afterlife.
天国 is the paradise or heaven where good souls dwell, the direct opposite of 魔界, the realm of demons and dark forces.
Compound of 魔 (ma, 'demon, evil spirit') and 界 (kai, 'world, realm'). The exact historical origin is uncertain, but it is a straightforward Sino-Japanese formation.