Wu Jen are an Arcane Spellcasting class present on the more Chinese flavors of Dungeons & Dragons. They combine the traditional Wizard spellcasting abilities with Chinese-flavored Elementalism (Earth, Water, Fire, Wood, Metal) and often have Monk-esque Ki Powers, but also have a lot of weird taboos that they must obey or else they lose access to their magic, kind of like Paladins, or pre-4th edition Druids, Barbarians and Monks.
Basic Dungeons & Dragons
The Wu Jen appeared in BD&D in the Oriental Adventures by Gary Gygax. They were identical to Magic Users in the Players Handbook with the exception of Ki Powers that were gained as they advanced in level and a Taboo that must be selected at 1st level. Violating this taboo resulted "in the loss of spells, illness, or other evil events" (Oriental Adventures, pg 26). Most taboos were related to issues of personal hygiene such as bathing or the cutting of hair.
An upgraded version of the Wu Jen for AD&D was featured on page 54 of issue #229 of Dragon Magazine. This version of the Wu Jen, like its original counterpart, was essentially a sort of wizard-monk hybrid, using magic and martial arts training combined to serve as a rambunctious wu-shu style warrior-wizard.
The AD&D Wu Jen class is available to any race that can take the Mage (generalist Wizard) class. Requiring Int 9 and Con 12 to join, its key attribute is Intelligence, with a +10% bonus to XP gain for Int 16 or higher. Wu Jen use the wizard THAC0 progression and experience tables, but can use all weapons, can use magic items (bar armor and shields) available to both wizards and warriors, receive 1d4+1 hit points at first level, and 1d6 hit points at each subsequent level.
Wu Jen cannot be combined with kits or multiclassing.
A Wu Jen chooses one of five elemental schools at character creation: Earth, Air, Fire, Water or Nature. A wu jen can only learn spells from that school, and only gains 1 spell per level when leveling up.
Unlike a normal wizard, wu jen don't need to memorize spells ala Vancian Casting. They can cast any spell they know, whenever they like - at the cost of taking thrice the spell's level in damage when they cast. And, as a side-effect of the processes that let them do this, they halve the effects of magical healing (rounding up).
They still have to obey taboos, gaining 1 at first level and another at "every level divisible by five". Failing to observe a taboo costs the wu jen the ability to cast spells for 2d4 days.
Wu jen Trance like elves instead of sleeping. At 6th level, they gain the ability to spend 1d6+2 hit points in order to bump up any one physical attribute by +1d 4 points for 2d4 rounds. They also use the Exceptional Strength rating for fighters. Boosting Constitution does not grant bonus hit points. This physical buff can be used once per round, so long as the wu jen has the hit points to spend.
At 10th level, a wu jen begins attracting 1d4 pupils, all 1st level wu jen.
Finally, at 12th level, at the cost of 1 hit point, a wu jen can leap up to 30' in any direction they choose.
D&D 3rd Edition
Mimicking its introduction to the realms of Dungeons & Dragons, the Wu Jen first appeared in 3.x in the Oriental Adventures D20 sourcebook, before getting reposted with maybe some cleaning up in the Complete Arcane book, alongside the Warlock and the Warmage.
The main tweak was the added focus on Wu Xing inspired elemental magic; Wu Jen had a very specific list of spells they could use, dividing them up into Universal, Earth, Fire, Metal, Water and Wood categories. This was important because at 6th level they got to pick one of the five elements for "Elemental Mastery", which meant they had +2 caster level with spells from that category (as well as from the Universal category).
Beyond that, their big thing was the ability to pick one spell every 3 levels and apply a "Spell Secret", permanently applying the effects of one specific metamagic trait (Enlarge, Extend, Still or Silent) to that spell whenever they cast it.
They still had to obey their taboos, but violating one just cut off their ability to cast spells for the rest of the day. Which was good, because they picked up one taboo at 1st level and then a new one each time they gained a Spell Secret. The silliness of these taboos was lampshaded in the book:
- Can't bathe
- Can't cut hair
- Can't eat meat
- Can't touch a dead body
- Can't drink alcohol
- Can't wear one or more specific colors
- Can't light a fire
- Can't sit facing a certain direction
- Must make a daily offering of some kind (food, flowers or incense most commonly) to spirit patrons
They have some unique spells that make them inclined to become Gish. These include Giant Size (Increases your size a lot) and Body Outside of Body (Create copies of you that can't cast spells or use magic items, but retain your martial skills).
Like Shugenja and Pathfinder's Witch, Wu Jen are Tier 2 despite being prepared casters. This is because their spell list, which isn't really expanded elsewhere, lacks enough potent spells to trivialize everything, even if you know its coming.
There's no specific Wu Jen class in Pathfinder, but one could probably pull it off by taking one of the five Wu Xing-appropriate Elementalist archetypes (Earth, Fire, Water, Metal, Wood) and giving it some more Chinese flavor.
D&D 5th Edition
The Wu Jen reappeared in 5th edition as one of the six subclasses presented for the full Mystic class, which appeared on March 13 2017. Like most of its fellows, the 5e Wu Jen is ultimately based on one of the original AD&D schools of psionics - specifically, the Psychokinesis school, using psionic magic to manipulate the various elements. This is represented both by its list of associated Disciplines and by its class features, with the former directly manipulating various elements and the latter boosting its ability to interact with the elements both offensively and defensively.
At first level, it gains 2 additional Disciplines taken from the Wu Jen List - an array consisting of Mastery of Air, Fire, Force, Ice, Light & Darkness, Water, Weather, and Wood & Earth - as well as free proficiency in any two skills from a list consisting of Animal Handling, Arcana, History, Insight, Medicine, Nature, Perception, Religion, or Survival.
At 3rd level, it gains the Elemental Attunement feature. Essentially, if you haven't maxed out your psi limit, you can spend an extra psi point to ignore damage resistance when making an elemental psionic attack.
At 6th level, in a mythology gag to its origins as a Wizard variant, its Arcane Dabbler feature lets it take up to three 1st, 2nd or 3rd level wizard spells. It can fuel these by burning psi points to equate to spell slots; 2 points for a 1st level slot, 3 points for a 2nd level slot, 5 points for a 3rd level slot, 6 points for a 4th level slot, and maxing out at 7 points for a 5th level slot.
Finally, it finishes off its unique class features with Elemental Mastery, where it can boost a damage resistance it has to damage immunity for a turn by spending 2 psi points in reaction to taking damage of the appropriate type.