demon king

Immense Durability: The Demon King's physical resilience and durability are immense, being completely unfazed by Wild's Guruguru Wild attack. His resilience and durability likely contributed to his ability to easily survive inside Purgatory despite its extreme environment.
Immense Strength: The Demon King also has monstrously massive levels of physical strength, effortlessly flicked Wild away with a great force that tore apart the ground into it using a single finger when the latter charged at him quickly and later effortlessly stopped his attempts to drill him with only two fingers before effortlessly throwing him away.
Master Swordsman: Despite his large size, the Demon King has been seen to hold considerable levels of great agility and speed. Combined with his vast levels of skill and experience, the King can effortlessly wield his giant broadsword with a single hand with extreme finesse, able to deliver devastatingly fast and strong slashes that can effortlessly cut through landscapes. This even easily pressured the extremely fast Ban and Meliodas into the defensive and cut off Ban's arm while the latter himself was barely able to dodge the attack. Later on, when he engaged Meliodas in a one-on-one swordfight, the Demon King easily had the upper hand and quickly defeated him.
High Intellect: The Demon King is shown to be considerably calculating and intelligent, quickly realizing that giving just a single demon half of his power would mean threatening his rule and chose to divide his power into ten pieces so he can still rule the vast, chaotic Demon realm without allowing the challenge. He eventually chooses to use a creature from Purgatory to keep watch over Meliodas after realizing that creatures from the living world are far too weak and less long-lived. From realizing that Meliodas too does not remember anything related to Estarossa, the Demon King realized that all the memories held by the demons on Estarossa were forged and quickly deduced that it must be the demon Gowther due to the nature of the former's powers.

Abilities
Main article: The Ruler
The Ruler「支配者ザ・ルーラー Shihai-sha (Za Rūrā)」: The Demon King's eponymous magic ability that also goes by his own name. It allows the user to invert all the attacks sent at him, including Meliodas and Ban's Full Counter and Snatch abilities. All attacks and weakness are converted into healing and fortification[8]. Even the effects of others' powers are completely reversed; per example, when Ban tried to steal Demon King's energy, he was forced to give his own to the ruler. However, as The Ruler functions on the concept of magic inversion rather than magic absorption, any magic meant to strengthen him or increase his power will instead weaken him, as shown by Ban. Despite this weakness, he can easily overcome it by deactivating his own power of inversion.
Tansoku no Kenjin「嘆息たんそくの賢人けんじん Tansoku no Kenjin; literally meaning "Sighing Sage"]: the Demon King make a disembodied head out of his clawlike protrusions, that shoots out a blast of energy and destroys around the area.[10]
Setsuna no Inja「刹那せつなの隠者いんじゃ Setsuna no Inja; literally meaning "Hermit of Moments"」: the Demon King make a full-body creature out of his clawlike protrusions, it is able to spin his sharps tendrils to great speeds to attack his enemies.
Gokumon-tou「獄門刀ごくもんとう Gokumon-tō; literally meaning "Hell Gate Blade"]: the Demon King uses his magic power to create a gigantic sword of darkness.
Beast of Darkness: the Demon King summons a huge many armed creature.[13]
Death Zero「虚無デスゼロ Desu Zero; literally meaning "Nothingness"」: The Demon King engulfs the opponent in his darkness, crushing them with a great pressure.
Main article: Commandment
Commandment: After dividing his power in half, the Demon King granted ten fractions of this power portion to ten elite warriors, which delivered a unique curse to whoever breaks a rule specific to each of them. Notably, these curses are active even when their holders are drained of magic, so they can be perceived as different compared to normal magic. This increases the power of whoever is wielding them and if they were to be bestowed to individuals outside the Demon Clan, the Commandments would turn them into Demons as well. It cannot be opposed even by its wielder; only those who also possess a specific immunity such as another Commandment or have received Divine Protection from the Supreme Deity are immune, while the curses will only be lifted if the victim defeats its wielder or at the victim's death. The Commandments can be withdrawn from their wielder if they give their consent, if they are powerless enough to be unable to fight or if they are dead. Curses: The Demon King is exceptionally accomplished in the use of curses, being capable of developing and casting highly unique yet powerful and durable spells. Perpetual Reincarnation「literally meaning "Perpetual Samsara"」: A durable curse that causes the victim to reincarnate when they die and forget everything they did in their past lives. If the victim is given enough information regarding their true identity, their original race's traits and fragments of their memories will slowly come back. Once all their memories are restored, the victim will unavoidably die in three days in some way. If used on a Goddess, it causes them to reincarnate as a human but retaining their original power. The only known victim is the Goddess Elizabeth and the only way to remove the curse is to obtain a level of power equivalent to the Supreme Deity or the Demon King, if not superior.[15]
Nature Manipulation: The Demon King has powerful control over the elements, causing a fog of death as well as fire and lightning capable of killing all the inhabitants of Belialuin along with the Supreme Deity. On his own, the Demon King was able to send out several strikes of thunder at Meliodas with a single hand movement.
Manipulation Immunity: It can grant an individual with immunity to the possession and brainwashing powers of the Goddess Clan. This negation spell was used on Merlin.
Power Transference: The user can transfer different powers to others, including all the secret techniques of the Demon Clan and lending a portion of his power to others. This supporting spell was used on Merlin and Zeldris when the younger demon became his representative and obtaining the power God.
Unknown Telepathic Power: The Demon King has the ability to telepathically communicate with his representative even while still sealed in Purgatory. Through unknown means, the Demon King also can see through the eyes of creatures, even from other worlds like Hawk. According to Meliodas, his father used to keep watch over him to make sure he carried out his duties via the creatures around him.[16]