Sohei

Historic Sohei were the fighting-monks of Japan who lived in monasteries and devoted their lives to physical and spiritual perfection.

However, unlike their Kung-Fu / Buddhist counterparts in China, Sohei were quite militant and did not take the same vows of non-violence and could be more closely likened to western crusading knights. Suffice to say this did not go down well in feudal Japan who were not particularly given to religious crusading the same way as Europe was, they were eventually destroyed by Oda Nobunaga in the late 16th century.

They were also similar to real-life Shugenja but were more closely associated with organised monasteries, rather than individual hermits or court advisors.

3rd Edition D&D

Found in Oriental Adventures, the authors could have been tempted to just use the Monk class to represent them like they could also have done with the Shugenja, but Sohei of their era trained in multiple weapons and wore armour, so they required their own class which appears to function very similarly to cross between a Monk, a Barbarian and a Paladin.

Just like the Monk, it is required to be Lawful, it has the same BAB progression and gains a couple of abilities such as Deflect Arrows, Defensive Strike (which allows you to counter an opponent who misses you) and immunity to sleep and stunning effects.

Similarly to the Barbarian, it gets the ability to Ki Frenzy which is like the lawful version of Rage that grants STR & DEX bonuses as well as the Flurry of Blows that a Monk would normally get. In addition to this, the Sohei eventually gains damage reduction, the ability to remain conscious when they should normally be disabled, and Mettle which is like Evasion, but works for Fortitude effects.

From the Paladin the Sohei gets a reduced divine spellcasting list, which contains mostly buffs and debuffs but no healing or offensive spells.

What hurts the class is the lower BAB progression, since your Flurry of Blows only works while in Frenzy, so you can't really become the killing machine of either of the three comparable classes, as well as your lower hit point dice and inability to cast healing means you can't tank things as well as a Barbarian or Paladin could, despite being able to wear Heavy Armour.

However, it makes for quite a good stand-in for any of these classes if your group does not already contain one (probably not a Paladin since those are usually banned in Oriental settings) and would make a good alternative to a barbarian, especially in more civilised settings.