The Cenobites of the Pleasure in Pain, also known as the Hierophants of the Order of the Gash, is a shadar-kai cult of personality gathered around an entity known as the Architect of Pleasure and Pain, or alternately, the Engineer of Pleasure and Pain. (Occasionally, this entity is called "Leviathan" for unknown reasons.)
The origins of this group are unknown. Anyone's best guess is that the Cenobites (or Surgeons, as they are also known — they seem to be quite enamored of epithets) form from a confluence of three factors. The first, is the shadar-kai themselves — when the shadar-kai first arrived in the Shadowfell, they found that they would fade away without strong emotions. As such, they took to the extremes of living to maintain their own identities.
The second factor is a now-disbanded Sigil faction called the Society of Sensation. Before they disbanded after the Faction War, the Sensates believed in experiencing all things to achieve enlightenment. Despite their reputation as hedonists, they sought to experience all available experiences — positive and negative. Although the Sensates as a group disbanded after the Lady of Pain's decree, some keep their traditions alive. It is not precisely clear how a group of shadar-kai came across the beliefs of the Sensates, but the shadar-kai would easily take to their worldview of empirical thought and experiential enlightenment.
The third factor is the Architect itself. This entity is apparently a somewhat potent baatezu, as those shadar-kai pledged to it are almost universally warlocks. The Architect appears as a striking, sensual male humanoid with pale skin and leather clothing. This figure appears to be constantly in a mixture of ecstasy and agony from the various hooks, pins, and piercings worked through its flesh.
Whatever their origins, the Cenobites typically keep to themselves in their own demiplane forged partially from the Shadowfell and partially from the Nine Hells. However, they sometimes emerge to explore the boundaries of experience, and to proselytize their experiences to others. While they consider themselves evangelists and explorers, most communities encountering the Cenobites consider them to be invaders and fiends of the worst sort, arcane torturers who typically leave their victims dead or worse. The Marquis d'Ennui of Sorgtomb has a standing bounty on any Cenobites found in his domain.
The Cenobites usually travel between realms through the use of arcane puzzle boxes provided by the Architect.
Unbeknownst to the Cenobites, the Architect is actually an aspect of Belial, Lord of the Fourth and Lord of Pain and Sufferings. Belial is using the Cenobites as a cult to further his goals, and also as a militant order in case he needs to counteract any plans of his scheming daughter, Fierna. It is possible that Fierna controls a group similar to the Cenobites.
Playing a Cenobite
Cenobites are universally shadar-kai (Dragon 372, page 5) and almost always have the Sensate theme (Dragon 414). They are usually of the warlock class (Player's Handbook), although some may not be directly bound to the Architect by contract. Some Cenobites learn how to use a spiked chain; those who do take the Spiked Chain Training Feat (Dragon 372, page 11) and may take the associated Novice, Expert, and Specialist Feats as well.
Cenobites' warlock powers usually manifest as barbed chains appearing from their bodies or nearby shadows.
It is, of course, possible to play a former Cenobite (much like the typical repentant warlock character). In such a case, the character is likely being hunted by his or her former comrades.
NPC shadar-kai appear, among other places, in Monster Manual and Monster Manual 2.
Enterprising DMs could use probably adapt the Cenobites for use in other editions of D&D. The shadar-kai appear in the 3e Fiend Folio, while warlocks appear in Complete Arcane. It is also possible that they could created using the rules for magic-users, clerics, elves, and suchlike in earlier editions.