Wu Jen

The only acceptable Wu Jen

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.

Advanced Dungeons & Dragons

The Wu Jen appeared in AD&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.

D&D 3rd Edition

As in AD&D, 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

Pathfinder

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.