The Demon Realm is the homeland of the Demon Clan.
Not much is known about the Demon Realm, except that it is the birthplace for most members of the Demon Clan. According to the Lady of the Lake, the Demon Realm was created along the Demon Clan by the Demon King, after this was given birth by Chaos.
The Demon Realm is host to beasts of destruction known as Indura, which happen to be the true form of most upper-ranked demons. However, not all upper-ranked demons can withstand said transformation, while those that can only use it as a last resort due to the costly sacrifice that has to be made. According to Merlin, The Demon Realm is a polluted and corrupted land covered in miasma in which no other race than the Demon Clan can survive, as the other races are prone to rot and decay there, physically and mentally.