The Demon King is a background antagonist from Seven Deadly Sins manga, who's yet to make an appearance in the anime of the series. He is the ultimate evil and the leader of the Demon Clan. He is a background character in Worst Hero And Villain War Ever lore.
Worst Hero and Villain War Ever
The Demon King is arguably the most horrifying entity in the Earthrealm WHVVWE takes place in, but thankfully, he has been sealed away...for now. He was once one of the most powerful entities ever, who conquered many universes. The Demon King is said to be the very first Demon, and one of the few real threats to Elder Gods, alongside Shinnok and Kronos.
Demon King enjoyed collecting powerful items from universes, in order to boost the Demon Clan's power. He happened to stumble upon a particularly powerful weapon, known as Crystal of Souls. Loki, the owner of the Crystal, instead gave him a false crystal, which launched him to a dimension known as The Black Hole. He was sealed in the realm and couldn't escape since. The Gatekeeper, who later received the power over The Black Hole, was warned by Loki himself to not use it too much, so that he wouldn't accidentally set Demon King free.