The Demon Realm is the homeland of the Demon Clan.
The Demon Realm is host to beasts of destruction known as Indura, the true form of most upper-ranked demons with seven hearts. According to Merlin, The Demon Realm is a polluted and corrupted land covered in miasma in which no other race than the Demons can survive in, as all others are prone to rot and decay physically and psychologically.
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 alongside the Demon Clan by the Demon King, after he was given birth by Chaos.
After Meliodas became the new Demon King of his own power, he planned to return to the Demon Realm since his presence was causing massive weather disruptions across Britannia. Elizabeth planned to go with him so they could still be together, reassuring that with her Goddess powers can help her survive in the Demon Realm.
Unfortunately, that plan came to an abrupt end when the Demon King turned out to be alive after possessing Zeldris's body and Elizabeth's curse of Perpetual Reincarnation has returned.