The Hierarchy species was an alien race from a region of space in the Delta Quadrant. The overarching polity or sociopolitical entity to which the species belonged and comprised is called the Hierarchy (from which the species' unofficial name is derived). The true name of this species is unknown to the Federation. What is known, however, is that their species was highly adept in the skills of espionage and remote surveillance.
Physically, the species was humanoid with dull brown leathery skin and generally nondescript features. Their heads were lumpy and hairless, with few distinguishable facial features, having no eyebrows, lips, or nose–though nostrils are present. Their ears were similarly understated, marked only by subtle embossment around curved openings on the sides of their heads. There also appeared to be two vertical rows of at least eight raised pits running down the back of their heads. Additionally, members of the Hierarchy species possessed a rotund appearance, their relatively unathletic build compared to other humanoid species suggesting a sedentary lifestyle.
They regularly used surveillance on passing ships to plan future raids. This was done by special surveillance ships, hiding in nebulae for example, which used highly advanced (compared to other species) sensors to assess the tactical risk of attacking passing vessels and the value of their cargo. The Hierarchy's cloaked ships also allowed them to sneak up on targeted vessels; after which they would execute a preselected attack pattern, such as the Type 3 or Type 4 stealth assault. Their attack patterns are numbered according to their increasing levels of energy expenditure. The Hierarchy was meticulous when it came to the use of energy and resources on its vessels. Every planned use was to be reported and consulted with the Hierarchy to maximize efficiency. (VOY: "Tinker Tenor Doctor Spy")
The Hierarchy itself was regularly consulted by crews of its ships using a special communication system. The situation was detailed on a sort of PADD then entered in a slot next to a cylindrical horizontal display. Changing colored patterns would subsequently appear on the display to indicate the Hierarchy's decision.
The social and working ethics of the Hierarchy differed dramatically from Starfleet in that every crew member had only a single duty and work station, minimal social interaction, and limited access to the rest of the ship. However, some members of the Hierarchy were known to leave their territory and explore other areas of space. (VOY: "Renaissance Man")
A Hierarchy survey vessel was one of the many that got trapped within the subspace anomaly known as The Void. The Hierarchy crew joined Captain Janeway's alliance, where they uncovered an imminent threat with their espionage techniques. (VOY: "The Void")