Ultimate Skill

Ultimate Skill (究極能力) is the evolved form of Unique Skill and the final evolutionary stage of Skills in general.

Characteristics
Ultimate skills, as the name implies, gives the wielder ultimate power to control the very laws of nature. The only way to counter an ultimate skill is with another ultimate skill. That is the absolute, unbendable rule of the world.

If unique skills are born from one's individuality, ultimate Skills are born when unique skills are brought past their limit. That's why no ultimate skill is identical.

As the story progresses, it's revealed that through certain methods, ultimate abilities can be imparted to others who have not naturally awakened them. The only requirement, like natural ultimate skills, is that the recipient must be a spiritual lifeform to receive it.

Angelic Line
Angelic Line (天使系) are a family of ultimate skills directly created by Veldanava bearing the name of angels. Among them, the most powerful in effect are the Virtues (美徳系), Veldanava's original powers. After the world stabilized, he parted with all but the Lord of Justice, gifting them to certain individuals, and releasing the rest into the wheel of reincarnation, where they sprouted within the souls of the qualified. As they were too powerful, they were restricted to the unique skill level.

Although not directly stated, it can be inferred that the Virtues had dormant egos built into them during their creation. With the exception of Michael and Ciel, when Chloe was temporarily placed under Feldway's Ultimate Dominion, her ultimate skill Sariel showed signs of a conscious, albeit weak, and responded to Feldway by affirming itself as separate from Chloe. These egos are presumed to awaken in response to certain stimuli. In Ciel's case it was Rimuru's desire for companionship; in Michael's case it was Veldanava's death.

All the ultimate skills of the angelic line are linked to the Lord of Justice, with control circuits directly built into them by Veldanava, placing their holders under the absolute control of the Lord of Justice.

Demonic Line
Demonic Line (悪魔系) are powers born from imitating skills of the angelic line. Among them the most powerful are the Deadly Sins (大罪系), skills derived from a person's fundamental desires. Though imitations, they boast the same potency as the angelic line. And just like their angelic counterpart, they awaken within the qualified as degraded unique skills.

Ultimate Skills
Rimuru Tempest:
 * Azathoth, God of Void
 * Shub-Niggurath, Lord of Abundance
 * Raphael, Lord of Wisdom (formerly)
 * Beelzebuth, Lord of Gluttony (formerly)
 * Veldora, Lord of Storms (formerly)
 * Uriel, Lord of Vow (formerly)
 * Velgrynd, Lord of Scorch (formerly)

Veldora Tempest:
 * Nyarlothotep, Lord of Chaos
 * Faust, Lord of Investigation (formerly)

Diablo:
 * Azazel, Lord of Temptation

Benimaru:
 * Amaterasu, Lord of Shimmering Flame

Testarossa:
 * Belial, Lord of Underworld

Ultima:
 * Samael, Lord of Deathly Poison

Carrera:
 * Abbadon, Lord of Destruction
 * Sandalphon, Lord of Judgement

Ranga:
 * Hastur, Lord of Starwind

Beretta:
 * Deux Ex Machina, Lord of Divine Machines

Masayuki:
 * Lord of Heroes

Luminous Valentine:
 * Asmodeus, Lord of Lust

Hinata Sakaguchi:
 * Fortuna, Lord of Odds

Yuuki Kagurazaka:
 * Mammon, Lord of Greed
 * Melchizedek, Lord of Dominion

Chloe Aubert:
 * Sariel, Lord of Hope
 * Yog-Sothoth, Lord of Space-Time

Zegion:
 * Mephisto, Lord of Illusion

Velzard:
 * Gabriel, Lord of Patience

Velgrynd:
 * Cthugha, Lord of Flame God
 * Raguel, Lord of Charity (formerly)

Milim Nava:
 * Satanael, Lord of Wrath

Dino:
 * Belphegor, Lord of Sloth
 * Astarte, Lord of Heaven

Rudra Nam Ul Nasca:
 * Michael, Lord of Justice
 * Uriel, Lord of Vow (formerly)

Guy Crimson:
 * Lucifer, Lord of Pride

Leon Cromwell:
 * Metatron, Lord of Purity

Kagali:
 * Melchizedek, Lord of Dominion (formerly)

Trivia

 * In the story, the characters pronounce Ultimate Skills using only the being they reference and not the entire names used in the wiki and translations. Meaning, it's "Ultimate Skill Beelzebuth" not "Ultimate Skill Lord of Gluttony Beelzebuth." This is due to the Japanese texts themselves. (See: Furigana and Ruby Characters)