Black Numbers

Black Corps is a unit of Tempest composed solely of elite demons that serves Rimuru under the leadership of Diablo.

They are approximately 700 personnel consisting of Diablo's old rivals, their subordinates and unique individuals. After being summoned, these demons are made to inhabit/possess the demon dolls inside capsules developed by Tempest.

Background
It was an idea formed from the mind of Diablo, whom planned to form a force worthy of serving his master, Rimuru. Thus Diablo set off to the Underworld to search for potential demon candidates, laying waste to countless demons in his wake.

After gathering the suitable candidates, Diablo presented them to Rimuru upon his return to Tempest and summoned ten elite demons that he deemed worthy.

Organization
The Black Numbers is a group of 700 demons, currently composed of four named Devil Lords, 7 named Demon Nobles, 100 nameless Demon Chevaliers, and 600 other nameless Greater Demons (some evolved into Arch Demons).
 * Devil Lord Diablo - Founder and Leader
 * Lieutenant: Earl Venom
 * Devil Lord Testarossa -  Captain
 * Liuetenants: Duke Moss and Baron Cien
 * Devil Lord Ultima
 * Liuetenants: Marquiss Veyron and Baron Zonda
 * Devil Lord Carrera
 * Liuetenants: Viscount Agera and Esprit

History
After the Tempest tournament, Diablo returned to the Underworld with the intention of gathering distinct demons, some of whom were actually his rivals, worthy of serving his master Rimuru. Upon his return, he introduced 3 Arch Demons, 7 Greater Demons and the remaing 700 Lesser Demons. As they are spiritual forms, they thus used the dolls developed by Ramiris as their vessels in order to manifest in the physical plane.

The 3 Arch Demons -- Testarossa, Ultima, Carrera -- evolved into Demon Dukes after being named, and their incarnation to the doll was instantly completed, turning into real beauties.Their might is horrifyingly overwhelming while their strength is on a completely different dimension. The 7 Greater Demons: Venom, Veyron, Moss, Shien, Zonda, Agera, Esprit --- also evolved into Arch Demons after being named. One hundred (100) Greater Demons were born among the total of 700 Lesser Demons' souls that are lodged onto the cultivated demon dolls due to accumulating enough magic essence/energy for evolution from inhabiting the dolls. These demons are revealed to be powerful with abilities that surpass the Majins/Devils. Dozens of the Lesser Demons became Greater-demons-like and have abilities above A ranks.

During the rewarding ceremony after the invasion of the East Empire, Rimuru granted 100,000 souls to Diablo, who in turn evolved into a True Demon Lord/ Devil Lord, and in effect, his lieutenant, Venom, evolved into an Earl class Demon Noble. After Guy gave Rimuru 500, 000 souls in exchange of evolving Mirari and  Hirari into True Demon Lords, and Rimuru used them to evolve the rest of his Twelve Patrons. Testarossa, Ultima, and Carrera were further evolved into True Demon Lords/Devil Lords and was thus referred to us the Devil Lord Trio. As such, their lieutenants evolved as well into Demon Nobles, the 100 Greater Demons evolved into Demon Chevaliers (Knights) and the 600 Lesser Demons evolved into Greater Demons.

Trivia

 * It's Implied that Black Corps have enough power to Overwhelm and Conquer The Eastern Empires. (Excluding Velgrind)
 * The Black Corps has the fewest members, yet considered as the strongest unit in Tempest's military.