The Monk
The monk class is only available to the Graw (underdark elves) and the thri-kreen. Unless you start out as one of these races, a player cannot begin game as a monk but, you can still become one later on. This means that you have to find a mentor who is willing to teach you the art and discipline of being a monk. Graw often have complicated backgrounds that involve a lot of doubt and curiosity (a longing to know more about the outside world). So, be prepared to have an equally complicated career.

The Zealot
These brothers1 have gone past a normal devotion to their order. They are obsessed fanatics that have lost their reason and sense of self-control. In their unrelenting pursuit of the order’s supremacy over everything else, they consider this everything else as pure heresy and thus a target for their purging.

The zealot is extremely violent and can enter a stage of frenzy that imbues his body with an incredible strength and stamina. The zealot combines this with a perfectly honed skill in martial arts, making him a truly terrifying opponent. In order to become a zealot, the monk has to undergo an alignment shift. The character can no longer advance in monk levels and has to take levels in barbarian from now on. The alignment shift can come naturally (some monks are born zealots) or can be imbued through mantras and indoctrination2. Zealots constantly repeat these mantras to themselves even muttering them in their sleep. The only time when they don’t do this is when they rage.

Basically, Zealots are multi-classed characters that can only take levels in the monk and barbarian class. This is in a way a limitation, you can't select any other class but, you do get to keep all of your monk abilities and at the same time advance as a barbarian. This offers incredible combat potential. On top of that, this is good roleplay (although I doubt that many players would want a fellow player to become a fanatic and then oppose them in combat because they're not of the right faith.)

1: Only Graw brothers can become zealots; sisters are (considered) too well balanced in mind and body. No thri-kreen zealots have been known to exist. They reject the fanaticism that comes with the zeal. Also, because they don't live in constant war with other monasteries, they don't have the need to prove that they are the best.
2: This process is reversible but, will only be done in rare cases and is only possible if the number of barbarian levels doesn’t exceed the number of monk levels. If this is done, the character loses all ability to rage, all the other class related abilities are still usable. (improved movement, etc.)