Demon Realm

The Demon Realm is the homeland of the Demon Clan.

Background
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.

History
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.

Originally a vast and chaotic realm, this was conquered and placed under the control by the Demon King with the help of the Ten Commandments.

It also contains two moons

Demon King arc
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. With help from Elizabeth, the Sins, and even Gelda, Meliodas was able to drive the Demon King out of Zeldris and defeat him, and in order to truly destroy his father, he sacrificed his godly power to cancel out the Commandments; thus Meliodas no longer has to return to the Demon Realm and risk his power destroying Britannia.

Trivia

 * The Demon Realm is likely based off the concept of Hell from monotheistic religions and others such as the Underworld from ancient Greek mythology.