An Apostle (sometimes also referred to as an Acolyte) is a mortal being that has been granted eternal youth and considerable power by a Dark Lord in exchange for their servitude.
Apostles are created via a process known as a blood pact. During a blood pact, the prospective Apostle drinks the blood of the Dark Lord. In exchange for pledging their allegiance to a Dark Lord, an Apostle receives eternal youth and greatly enhanced power. Any creature can become an Apostle so long as the Dark Lord shares its blood with it, including Monsters, Haniwa and even Insects.
Apostles are unable to resist any order given to them by their master, this is a similar method to the blood control that the Demon King uses to command Dark Lords.
Typically, their combat power is enhanced far beyond that of a regular member of their species, and they often serve as combat personnel for their master. They do not receive the Invincibility Field, however, and can still be killed through ordinary means, though with much greater difficulty than most other creatures. In some circumstances, mental power or other abilities inherent in the individual are enhanced, making them more proficient in areas besides combat. Many Apostles serve more domestic purposes for their master, performing more mundane tasks such as cooking and cleaning. Other times, Apostles are simply made to act as pets for their masters, and aren't relied on for any particular purpose beyond company and entertainment.
Unlike with Dark Lords, there is no limit to how many Apostles there can be. A Dark Lord is free to contract as many individuals into becoming Apostles as he or she likes. There is, however, a very heavy cost for doing this. Because the Apostles draw their power directly from the Demonic Blood Soul of the Dark Lord, the more Apostles a Dark Lord has, the weaker the Dark Lord becomes and the more difficult it becomes to control each Apostle since the blood that controls them is spread thin amongst them. Dark Lords can revert their Apostles back to mortality at will, regaining the portion of their own power given to them in the process. Apostles are also capable of returning the blood they received from their master as well if they desire, unless their master explicitly prohibits them from doing so.
When their master Dark Lord dies, an Apostle is able to walk free from their control. However, as most Apostles are incredibly devoted to their master, they generally take it upon themselves to protect and watch over their master's Blood Soul until they regenerate from it. Because the blood pact is not technically with the Dark Lord themselves but with the Demonic Blood Soul, if another being consumes the Blood Soul and overwrites the will of the Dark Lord within it before they are able to regenerate, taking that being's place as a Dark Lord in the process, they inherit all of the Apostles controlled by their predecessor. If the Blood Soul is absorbed by the Demon King, or simply lost, the Apostle is freed from the command of anyone and is allowed to keep their enhanced abilities.
Lesser Apostles are individuals who are working with Dark Lords under good faith, without performing a blood pact. These individuals that do not make the blood pact do not receive any of the benefits that a standard Apostle would and are simply ordinary creatures in the service of a Dark Lord.
List of Apostles
Dark Lords without known Apostles