Onmitsukidō

"If you see your allies losing, that's your chance. Don't step between them. Take the enemy down from behind. And if the enemy is so strong you can't even do that, then just watch your ally be killed. That's what it means to be Onmitsukidō." - Suì-Fēng
The Onmitsukidō (隠密機動, secret tactics, called the Stealth Force in the English dub), also called the Secret Mobile Unit/Corps, Special Forces or Covert Ops, is the second largest organization that Shinigami join, and one of the three main branches of military in Soul Society.
History
Former Commander-in-Chief Yoruichi Shihōin with her guards
Although by nature an organization separate from the Gotei 13, the clan which possessed superior abilities as Shinigami among the Four Noble Families, the Shihōin Clan, filled the role of of the Commander-in-Chief throughout the generations, while also having their clan leader take the post of division captain within the Gotei 13. Having served her post as 2nd Division Captain the then Commander-in-Chief, Yoruichi Shihōin, had strengthened the image of the organizations direct influence of the 2nd Division, as every head of the five divisions have all been ranking officers within the 2nd Division.
Mission
The Onmitsukidō is one of the three forces under the direct supervision of the Central 46 Chambers. If the Gotei 13 operate openly, the Onmitsukidō patrols and conducts surveillance in enemy territory and carries out top-secret operations, such as assassinations and the execution of Shinigami who have broken the law. If the Gotei 13 are considered an exterior guard, then the Onmitsukidō is an interior, covert guard.