Society of Sensation
The Society of Sensation, also known as the Sensates, are a Faction in Planescape defined by their belief in the power of the senses and the value of experience. In short; the multiverse only exists in so far as you have experienced it through the powers of your own senses, and the purpose of existence is to experience as much as you possibly can. This is not simple hedonism; the Sensates don't approve of those who just want to wallow in indolent pleasure, and actually have a special prison where they abandon those who commit to the Faction only to indulge themselves. No, a Sensate is an explorer, somebody who wants to know everything by feeling everything, pleasant or unpleasant. They're not suicidal, they're aware that risking life and limb for a novelty is something to be carefully gauged, but they concern themselves with the now and regard inhibitions as a dirty thing. That said, they are not Chaotic Stupid; a real Sensate both respects the wishes of their comrades if they say they don't want to do something, and won't go putting themselves in blatant danger just for the sake of a novel experience. A Sensate isn't afraid to gamble with his safety if the pay-off is worth it, but they sure as fuck won't swallow lethal poison "just to see what it tastes like"!
A Sensate desires new sensations. He shouldn’t want to go into every local tavern, try every wine and pastry dish, and sleep with every barkeep. Granted, he may try this approach when entering a new plane, where everything’s different but back home it’s another story. As Sensates grow in ability and gain levels, most realize that whole realms of thought and emotion wait to be explored. One Sensate might set himself a goal of trying to feel all the different aspects of love or anger, for instance, while another might choose to experience all the nuances of verbal comedy - in every language she can find. As Sensates age, too, they naturally grow in acceptance of others and of all things, having seen and felt so much in their lifetimes.
Whilst they may be looked down upon by the more grim, stoic or ascetic Factions of Sigil, such as the Dustmen, the Bleak Cabal and the Mercykillers (the Doomguard have a weird love/hate sort of relationship with them), the Sensates are hugely influential, as they have the ultimate control over the city's entertainment business. One of the younger Factions, having only originated about 700 years before the "present" of Planescape's initial release, they stem from a tide when Sigil was much more lawful in its nature - restrained, repressive and dull. A band of chaotic souls took it upon themselves to start making their own entertainment, and news of their exploits spread; they founded the Civic Festhall, a circus maximus displaying oddities and one-of-a-kind occurrences from across the multiverse. It was during this time that the roots of the Society took hold, although it wasn't until the Lady of Pain demanded the winnowing of the factions that they fully matured into the form they take today.
The hidden goal of Factol Montgomery is to find a way for all beings in Sigil to experience a cerebral transcendence, freeing themselves of their physical bodies so that they can all explore the multiverse freely and without restraint, at one with thought itself. This plan is derailed by the conflict with the Fated faction, and forgotten after the events of the Faction War. It does mean that there are a lot of Sensate spies burrowed deep in most of the other factions before then.
Additionally, the Sensates have access to both a hidden tunnel from the Civic Festhall that leads to the archives below the Hall of Records and to a ring of portals leading to almost every known plane.
Joining the Sensates is... not entirely difficult, but not easy, either. The Sensates will take anyone. Any race, any class, any alignment, so long as they truly accept the Society's philosophy of seeking sensation and experience. This does mean that Sensate adventurers tend to flexibility over specialization; a Sensate will usually aim to be more of a jack-of-all-trades, as this meshes better with the Society's focus on broad experience; a Sensate fighter will become adequately skilled in many weapons rather than focused on one, whilst a Sensate wizard will usually favor a diverse array of spells - mages are far more common in the Society's ranks than specialist wizards. Sensates are also extremely prone to multiclassing, furthering their experience potential. The tricky part is convincing the Sensates that you believe their philosophy; to join up, a would-be applicant has to use recorder stones (magical memory-storing tokens) to contribute five worthwhile experiences to the Public Sensorium's library, each of which must be focused on a single sense - Sight, Sound, Smell, Touch, Taste. Alternatively, they can contribute a single experience that strongly involves all of the five senses. These submitted experiences are judged by a Sensate factotum, who has final say on whether or not the would-be Sensate has passed the test. If they fail, they're told to go out and play in the multiverse a bit more, but they can come back and try again.
In Advanced Dungeons & Dragons, a Sensate gains Infravision to 60 feet and a +1 bonus to saves vs. poison and dice rolls vs. surprise. They also have a 10% automatic chance to detect if somebody is lying to them (20% if they're the same race). At 3rd level, they can perform a Sensory Touch 1/day, which lets the Sensate heal a person of 1d10 damage at the cost of suffering that same amount of damage themselves, which must be cured through natural rest. At 5th level, a Sensate gains the local history proficiency and identification abilities of a 1st level Bard.
Additionally, their connections to the Society come in financially handy; starting level Sensates begin with triple their default starting cash, and the Society is often more than willing to donate magical items or spells (either as scrolls or to a wizard's grimoire) to adventuring Sensates who promise to record the experiences of their latest quest.
In Dungeons & Dragons 3rd Edition, the Society of Sensation appeared in Dragon Magazine #287, with a Sensate prestige class. A further prestige class for 3E came in the Planar Handbook called the Ardent Dilettante, which was a nightmare to qualify for, since every three levels changed the requirements to continue taking levels in this class: absolutely requiring you to multiclass into an arcane spellcaster, a divine spellcaster, have lots of Martial Weapon Proficiencies as well as one Exotic weapon proficiency, lots of ranks in Perform and have a spread of ranks in other skills, have to visit up to six different planes, and eventually have to die and come back to life. As broad an unfocused as it seems, it definitely fits the Sensate theme of trying a bit of everything.
In Dungeons & Dragons 4th Edition, they again made an appearance in an issue of Dragon, #414, where being a Sensate became a character theme.