Great Demon Lord

The, or Beelzebub III, is the Demon Lord of the entire Demon World. He is a former ruler of one of the Demon World's countries prior to its unification, thereby being a member of the Seven Deadly Sins.

Appearance
The great-demon lord seemed to always use a helmet with a pair of horns on its sides and has long green hair along with a black cape. He appears on screen taken from an angle behind him, rarely having his frontal view seen. In Episode 53, it can be seen that he keeps a long fringe that masks his face, hence keeping the mystery revolving how he looks like intact.

Personality
The Great Demon Lord is a very stupid and forgetful king of Beezlebyute, and he wants to destroy the Human World because ''they breed like roaches. ''He loves to sing karaoke, he is also the father of both Lord En and Beelzebub IV. His wife, Iris, referred to him as the embodiment of a deadbeat husband showing that he is very irresponsible. Indeed, he ordered his infant son, Beelzebub IV, to go to the human world with his wet nurse, with the objective being the destruction of the Human World. He forgot that he made this decision within a day, and only then realized that Beel was just an infant so he could not carry it out so quickly.

Despite his portrayal as such throughout the story, he redeems himself slightly by the end of the series. He reveals that the reason he ordered the human world's destruction was because of people like the Solomon Company's elders. He apparently rescinds this order following Satan's defeat, smiling at the sight of Oga and Beel surrounded by their comrades.

Perhaps the Overlord's only real "Demon Lord"-esque action is at the end of the series, where he appears and destroys the entire Solomon Company and defeats its elders by himself. He attributes it to revenge because "My wifey came back all beaten", indicating that despite all his flaws and horrible personality, he cares deeply for his wife and won't tolerate her being hurt.

History
The Great Demon Lord was once a ruler of one of the Demon World's countries. However, he would eventually unify it and become the sole ruler of the entire realm. His reign as Demon Lord was challenged once when Lucifer, also a former ruler of one of the Demon World's countries, fought against him for possession of the royal throne. Though he was victorious, the conflict led to great turmoil in the realm.

Mobichi Arc
Behemoth mentions that he made a contract with the Great Demon Lord, though he does not expand on what they are. However, according to the contract that they made, the tissues that the Great Demon Lord sent to Tatsumi Oga were made to summon a Demon from Behemoth's Pillar Division after a specific ritual is made.

America Arc
While Hilda was temporarily in the Demon World, the Great Demon Lord gave her an item that she was supposed to bring to his wife Iris.

Powers and Abilities
Expert Artist: The Great Demon Lord is a skilled artist as he painted an exact portrait of his wife, Iris, to the very last detail though he apparently believes it's not good enough as he hides it away with creating an artificial demon to guard it.

Immense Demonic Power: As a member of the Seven Deadly Sins and the overall ruler of the entire Demon World after its unification, the Great Demon Lord is an immensely powerful demon and the most powerful known character in the series, as he triumphed over the other Seven Deadly Sins. Evidence of his great powers is that his two sons are able to cause widespread and colossal damage despite them being very young and having undeveloped powers. It's said by Hilda that his battle with Satan many years ago had split the entire Demon World in half. Satan himself remarks that the clashes of their strongest attacks were able to cause several calamities. A testament to his power is the fact that he is the sole member of the Seven Deadly Sins to avoid imprisonment by the Solomon Company while the other six were captured and when he confronted the leaders of the company he easily dealt away with them.

Electrokinesis: The Great Demon Lord possesses the ability to generate and manipulate electricity which is no doubt inherited by his younger son, Baby Beel. Though his is more developed and can cause enormous damage instantaneously while Baby Beel's electricity needs to be built up to have the same effect. Artificial Demon Creation: The Great Demon Lord has the power to create artificial demons like Tamapochi. It is currently unknown how he creates them. his artificial demons are made with specific purposes that serve their roles even after long periods of time of not meeting their creator.
 * Lightning Cannon: The Great Demon Lord can fire his electricity through the palm of his hand much like how Naga and Jabberwock does with their demon energy. His lightning cannon can easily destroy a building on the same scale as Oga and Baby Beel's undeveloped Zebul Finisher in their initial Super Milk Time state. His control over the blast of his attack was made instantaneously when he just lifted his hand forward while his son, Baby Beel, had build up hundreds of zebul emblems in his Super Milk Time to have the same result. An important point to note that is that these feats were all accomplished without a contract, which means his power was restricted, unlike his son's.

Although the full extent of the Great Demon Lord's power remains unknown, he is clearly the strongest character in the series. It is known that sometime during Oga and Furuichi's 3rd year at Ishiyama, he invited the other upstarts and Oga to the demon world. It is implied that he fought and defeated all of them relatively easily, as even Fuji and Takamiya remarked that his strength was absurd, and that the experience served as excellent training. He is also naturally stronger than the other Seven Deadly Sins, Satan included, as he is known to have triumphed over all of them and fought at least on par with Satan, causing many calamities throughout the demon world in the process.