- Web Novel
The specific requirements to obtain an Ultimate skill are quite vague. However, it seems that those of very high ability are able to develop them. The majority of Ultimate skill users are quite old (hundreds of years old). Although it is possible to quickly acquire an Ultimate Skill through becoming a True Demon King, this is not guaranteed. In order to acquire such a Skill, great willpower and control is needed. An ultimate skill strength is such that it must be defended against with another Ultimate Skill/Ultimate Gift.
A normal human's body can't handle the burdens of an ultimate skill's energy. Under normal circumstances, awakening is impossible. That's why the modified body of an “Otherworlder” or similar physiques needed. But, at the end of one's training people will "Evolve." Their race aside, there certainly is a change. In other words, from a human to a sage.
It is also possible to bestow skills close to Ultimate skills by one's master if he holds an Ultimate Skill by himself but in a much weaker form, known as Ultimate Gift.
In Chapter 190 of the web novel, as Rimuru pondered about the nature and existence of skills, unique skills, and ultimate skills, he explained how even if some (unique) skills can be grouped together because of their similarity, none are identical. He also explained that it's possible for skills with the same name to have different performances and rules.
As such, the name of a unique skill (and also an ultimate skill) can be thought to be a 'label' given to a 'container' that has the probabilities of obtaining numerous effects. This can be thought of as a game skill capable of exhibiting 10 effects total, but the player has only 6 while the other 4 are grayed out or invisible. An example of this would be Rimuru's Ultimate Skill Uriel and Chloe's Ultimate Skill Sariel both evolving from and having the properties of Infinite Prison.
Another example would be Carrera's Abaddon that added Sandalphon's effect "Divine Edge" to itself, meaning that Extinction King Abaddon already had the "aptitude" for Divine Edge but simply didn't obtain/unlock it until that time.
Official Skill Series
Currently, the only officially used terms for the types of Ultimate Skills are the Sin Series and Virtue Series.
Sin Series and Virtue Series
The Sin and Virtue Series' Ultimate Skills are based on the Seven Deadly Sins and the Seven Virtues. They are regarded as high-quality Ultimate Skills. The strongest individuals in the story either possess, desire, or at the very least have their attention drawn by one or more skills in these two series.
As such it may be no surprise that the Sin Series requires a unique process to obtain (this is not shown for the Virtue Series). First, a seed manifests in powerful individuals exemplifying the sin in question. Once rooted, if the sin is then nurtured by epitomizing it through one's thoughts and actions, it may blossom into the Ultimate Skill. At that point, any concurrently budding seeds of the same type in other users will vanish instantly. Like all Ultimate Skills, only one user at a time may be in possession.
These two series exist in opposition toward each other. For example, Justice King Michael rules over the Virtue Series, but if a user possesses equal or more Sin Series than Virtue Series skills, they can resist Justice King Michael's influence. There is no equivalent ruler of the Sin Series.
Speculatively speaking, the natures of virtue and sin may represent some racial affinity. Namely, in how angels lack the will and are predisposed to a ruler for guidance, while demons are limited by evolution, perhaps thematically represented by a seed growing before realizing full power. Certainly, there are no demons with a Virtue Series skill nor angels vice versa; in fact, the skills may reverse if the racial alignment changes, assuming Dino's case (explained below) can be applied to the Sin and Virtue Series.
Two Ultimate Skills related to the above also exist: Sky King Luciel used by the Seraphim Dino, which transformed into Fallen King Luciel when Dino transformed into a Fallen. The first is not definitively explained, but the latter is a hybrid of Prideful King Lucifer and Wrathful King Satanael, hence their placement here.
Unofficial Skill Series
The following series can be organized within a consistent pattern, but aren't explicitly referred to in the text as a formal Skill Series of the story's universe. Although one in particular, the Lovecraftian series, probably could be viewed as a formal series for all intents and purposes, due to all of them except Hastur being the exclusive evolution of the angel and demon series skills. They represent the highest tier of Ultimate Skills.
This is by far the strongest series of Ultimate Skills, referencing deities from H. P. Lovecraft's mythos. They are attained by fusing two or more Sin and Virtue Series Ultimate Skills, by Ciel's skill upgrading abilities, or by bringing one's Sin or Virtue Ultimate Skill to the pinnacle of its usage. In the web novel, these seem to have an inferior "King" form, which can be upgraded into a "God" form, as is seen with Chloe O’Bell's Yog-Sothoth. There is also a "Lord" form, but their standing in the hierarchy of evolution is not expanded upon.
- Void God Azathoth (used by Rimuru Tempest)
- Abyss God Nodens (used by Guy Crimson)
- Flame God Cthugha (used by Velgrynd)
- Ice God Cthulhu (used by Velzado)
- Chaos King Nyarlathotep (used by Veldora)
- Space-Time God Yog Sothoth Hortz (used by Chloe O’Bell)
- Space-Time King Yog Sothoth (used by Chloe O’Bell)
- Harvest Lord Shub-Niggurath (used by Rimuru Tempest)
- Stellar Wind Lord Hastur (used by Ranga)
True Dragon Series
The True Dragons are powerful beings that may take almost any form. Rimuru Tempest was able to absorb two of these Dragons - Veldora and Velgrynd and was able to turn them into Ultimate Skills. As such, any of the remaining True Dragons are hypothetically convertible also into Ultimate Skills.
Weapon Lord Series
The Weapon Lord skill series confers mastered proficiency over a specific armament, according to the name of the skill. Beyond a technical skill boost using this equipment (which other characters seem capable of naturally obtaining through training), it seems there is a power boost associated with the use of that gear as well. These seemingly bland benefits contrast with the colorful and decisive advantages one otherwise expects from a skill belonging to the vaunted Ultimate class. In other words, these appear to be the least impressive Ultimate Skills—although being Ultimate Skills, which demand an extreme level from their user, their wielders are still superior to probably anyone lacking an Ultimate Skill or Ultimate Gift.
- Weapon Lord Axe (used by Aria)
- Weapon Lord Bow (used by Mai)
- Weapon Lord Hammer (used by Tolneod)
- Weapon Lord Shield (used by Zero)
- Weapon Lord Spear (used by Olca)
- Weapon Lord Sword (used by Arios)
- Weapon Lord Whip (used by Priscilla)
These mostly refer to archangels, fallen angels, demons, and other persons or beasts in Jewish/Christian writings.
- Dominion Lord Melchizedek (used by Kazaream)
- Illusion King Mephisto (used by Zegion)
- Mythical Beast Lord Bahamut (used by Kumara)
- Extinction King Abaddon (used by Carrera)
- Hell King Belial (used by Testarossa)
- Poison King Samael (used by Ultima)
- Sandalphon the Executioner (used by Tatsuya Kondou)
- Temptation King Azazel (used by Diablo)
While humbly short, this series contains two skills that are unlike other Ultimate Skills. Evil Dragon Lord Azi Dahaka is notable for its potential to evolve the user into an immortal True Dragon, while Creation Lord Ahura Mazda is the skill Veldanava used to create the universe itself and everything within it, a feat against which only a fully utilized Void God Azathoth, which had stored Nihility energy until the end of time and space, could debatably compare.
- Evil Dragon Lord Azi Dahaka (various users, most notably Vega and Zero)
- Creation Lord Ahura Mazda (used by Yuuki, later acquired by Rimuru)
These are used by the oni spirit executives of Tempest. They reference the god of the sun and storms, respectively below. The third god is of the moon, (Souei's Shadow Moon Lord Tsukuyomi) but is an Ultimate Gift bestowed by Rimuru rather than an Ultimate Skill.
Other Ultimate Skills
So far, only one standalone reference exists, a German legend of Faust who makes a pact with the devil for knowledge and pleasures.
- Investigation King Faust (used by Veldora)
- 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 Gluttonous King Beelzebuth." This is due to the Japanese texts themselves. (See: Furigana and Ruby Characters