Irrepressibles are the warrior elites that lead entire armies of the undead into battle. They rule alongside the Necromancers, sharing a common secret that keeps both castes unnaturally alive.
Along with Necromancers and Daemonologists, my 40k Undead army also has amongst it's ranks members of an even shadier cult - The Resurrectionist Cult of Osingoma. A select few who die on the battlefield can be reborn by way of dark, forbidden rituals. Others can have their natural lives extended artifically, even beyond that of long lived races such as the Eldar.
An extract from the Warhammer 40,000 Wiki explains it better...
"Rarely encountered but insidious in nature, Resurrectionist Cults ultimately seek to conquer the secrets of life and death itself. Some resurrectionist cults preach a doctrine of the Emperor’s triumph over death and the conquest of human weakness, while others entreat darker masters, pursue utterly forbidden sciences, or hide baleful xenos or warp-spawned influences at their hearts. Often they espouse the goal of attaining physical immortality for the faithful and will go to unspeakable lengths to obtain their ends."
"More so than even the Sanguinary Cults, these groups attract insane and desperate individuals to their ranks, those who have lost everything, become the most degenerate heretics or follow the wildest of deviant creeds. Some resurrectionists even practice ritual revivification to indoctrinate their members. The most extreme examples of such sects believe that the Emperor’s plan is for mankind to follow him into a blessed “immortality of the flesh.” They even claim that it is possible through the use of utterly forbidden archeaotech to free the Emperor from the Golden Throne to walk among his people, “dead-but-alive-everlasting” to quote the darkly renowned Credo Mortifex. Such cults are hated by both the Adeptus Ministorum and the Adeptus Mechanicus and must throw up a murderous veil of secrecy and superstition in order to survive".