Muta

Originally a secret project of a Majin who worked under demon lord Clayman for many years, This Unique Race he belongs to is later revealed to be of the Dullahan Race, a Being of the Spectre Race. He is offended compared to the strongest of the Majin in Clayman's army the Five Fingers. Rather than be in Clayman's army he was forced to be tortured non-stop since his body would be used to make a special healing potion with his undying blood. which ended up making him lose almost all his memories during this, so when he was able to escape he generally didn't know who or what he was, let alone if he had a name in the first place.  Later on, he was found by chance by Rimuru Tempest who happened to be looking into reports of a mysterious monster in the ruins near the sealed cave. By chance, Muta was able to speak with Rimuru who understood what had happened to him and gave him a name, granting him a home in Tempest to be a guard, and or bodyguard of visitors of high importance. 

Due Note: Muta Before being named, was known as a "Lesser" Dullahan, meaning he wasn't fully developed as he thought, after being named, he was a full Dullahan, and when Rimuru evolved into a demon lord ( aka Demon Slime ), He was granted a gift thanks to the Harvest Festival: He had Evolution as well, to a Rí Dullahan. Offend, Most Dullahan normally do not evolve to this level of power, without more than 1,000 years of training. This is considered the Ultimate Evolution of all Dullahan, the King of them in legends of it. 

It is also important to Note; that Dullahan is noted as a Combination-Type Spectre, Due to being the fusion of a Superior/Greater Elemental and an Undead Body. 

Muta’s Unique Skill is known as "Magician" which later evolves after Rimuru's Harvest Festival to the Ultimate Skill "Merlin, Lord of Magus".

Added, with the Instinctic Skills of a Dullahan which include a "Split Mind" skill, as well as "Phantom Limb", Thought their "Self-Regenation" Skill, is unique in the case if they lose a limb they can't grow it back, but rather it can be closed up fully with scarring that can successfully be controlled still with the Phantasmal Limbs Skill as well.

List of Skills/features of skills

Unique or Ultimate Skill(s)

Merlin, Lord of Magus ( Original Unique Skill; Magician ) - This Ultimate Skill, allows Muta to instantly understand various ways to use a magic or set of spells within that magic. Additionally, he can combine aspects of magics to create unique spells or even brand new unique magics altogether, This skill allows Muta to also use said spells without needing to say their name, nor use staves to cast them.