Demon Realm

The Demon Realm is the homeland of the Demon Clan.

Background
The Demon Realm is host to beasts of destruction known as Induras, 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 other creatures are prone to rot and decay there.

It also contains two moons.

The Demon Realm is connected to Britannia through an ancient relic whose holes connect to specific locations in Britannia. One of them is God's Finger. The relic has a filtration system that eliminates any possible obstructions, including corpses or intruders.

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.

The Demon Realm host a gigantic beast called the Behemoth. Although it is normally peaceful and blesses Demons with its life force, any major disturbance turns it into a rampaging monster that devours the life of everything in its path. However, the Demon King was able to put it to sleep with his vast power.

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.

The Seven Deadly Sins: Cursed by Light
Months after the end of the Holy War, the Demon Realm is invaded by the second Fairy King, Dahlia, and the giant craftsman, Dubs. The two kill many of the demons who try to oppose them, including an Indura of Retribution, and dispose of the miasma with the power of the Supreme Deity.

Dahlia and Dubs are confronted and ultimately defeated by the Demon King's sons, Meliodas and Zeldris. After their battle with the Supreme Deity, Zeldris return to the Demon Realm as the new king.

Four Knights of Apocalypse
16 years later, the Four Knights of Apocalypse and their allies are guided to the Demon Realm by a demon woman named Ioladio Wellweger, with the goal of find the pathway to Camelot inside the Demon Realm. The pathway is revealed to exist in the back of the Behemoth.

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.