Centuries ago, possibly before the Great Upheaval that reduced the Factions to the fifteen we know today, the Incanterium was one of the most powerful of all Factions. Their philosophy was simple: the true key to the multiverse and all its power was arcane magic. By mastering the arts of arcana, one could master everything, because magic controlled everything. What shaped the planes? Magic. What allowed portals to function? Magic. What controlled time, gravity, weather, entropy, love? Magic, each and every time. By becoming one with magic, one could become the master of everything. Meta as this philosophy may sound, because don't forget that this was Advanced Dungeons & Dragons and wizards were pretty much king, it worked. From their Tower Sorcerous, a mighty fortress dedicated to harvesting magical lore and objects from across the planes, they were increasingly the ones actively running Sigil. Until one day the city woke up and found that the Tower Sorcerous was gone, and almost every member of the Incanterium had vanished with it. Most just shrugged, figured that the Lady of Pain had mazed them all for threatening her control of Sigil, and went on with their life.
Except the Incanterium isn't entirely gone. And maybe it wasn't entirely the Lady's fault. See, those Incantifers - or Incantifiers, according to 3e - have undergone some changes. That hunger for magic that drove them before has become a very literal hunger; they eat magic, sucking the juice out of magical items and consuming spells or magical attacks used against them by other beings. They need it to survive, and they don't intend to give up the ghost just yet...
Most incantifiers allow their bodies to wither and look aged, unless they have a vain streak, but in truth they're immortal so long as they keep feeding. So watch out for ancient-looking cutters with silver eyes when you're planewalking. Because Incantifiers don't take kindly to insults or being delayed.
In 2nd edition, only stats for incantifiers as monsters exist. Incantifiers have the stats of humanoid mages; "younglings" are 9th to 14th level, whilst the original incantifiers average 15th to 18th level. They're usually specialists (most commonly wild mages, transmuters, conjurers and evokers), always carry a trove of magical items for protection and utility, and carefully protect themselves with long-lasting and/or permanized defensive/utility spells like stoneskin, protection from normal missiles and detect invisibility. Additionally, because their whole schtick is having access to unique magic, the DM is encouraged to give them spells from sourcebooks, Dragon Magazine or even homebrewing that aren't availible to PCs.
You have been warned, in other words.
But, incantifiers have tricks beyond that. Firstly, because they're now pure magical energy inside of a faux-flesh shell, their physical attributes are heightened; all incantifiers count as having a Strength of 18/51 and a Dexterity of 18. As an added bonus, they don't need food, drink or sleep, don't need to breathe, and are immune to nonmagical extremes of temperature or environment. More importantly, they absorb spells, spell-like effects and the effects of spell-projecting magical items. With a Spell Resistance of 20% + 5% perlevel over 9th, if they manage to nullify such an effect, then they absorb it; this heals them 1 hit point per spell level absorbed and lets them cast spells without forgetting them, sort of like having a permanent Absorption Rod on their person at all times. The only magical effects they can't siphon off are magical weapons and antimagic areas. And even if they fail to absorb it, they can still make their normal saving throws.
As stated above, incantifiers need to feed this hunger for magic. If they don't consume spell levels equal to their experience level every month (spells are worth their own spell level, magical items are worth 1 spell level per 500 XP value), then they lose an experience level. If this drops them to level 0, then they're dead, permanently.
The Incantifiers reappeared under their new name in Dragon Magazine #339, this time upgraded to a prestige class. 5 levels long, this PrC requires access to 6th level arcane spells, 14 ranks in both Knowledge (Arcana) and Spellcraft, and access to an existing incantifier in order to complete the transformation ritual.
3e incantifiers don't get any physical boosts, and aren't immune to environmental extremes anymore. They do, however, retain their eternal lifespan and lack of need to breathe, eat, drink or sleep. Morever, they can now explicitly cast spells on themselves in order to recharge depeleted spell slots (basically a way to tactically rearrange their spell-lists on the fly) or heal themselves.