This post contains spoilers for various Mists of Pandaria quests.
Alone among all pandaren, the Shado-Pan know the true horrors that lurk beneath the gentle earth that bears the crops. The Shado-Pan stand watch against the hordes of the mantid, give battle to the yaungol, and most importantly stand against the Sha. And in so doing, they must make a terrible sacrifice. Each member of the Shado-Pan is hardened by the trials she or he must endure, but not merely in a metaphorical sense. The horrors of war against the Sha reach into their very being, so that they alone among all pandaren are carved and cut away by the Sha and their presence. Their very souls are scarred by the battles they endure against creatures of pure hatred, fear, anger, doubt, despair and violence. In essence, the Shado-Pan endure this great horror so that the rest of Pandaria might be spared it.
Make no mistake - the Shado-Pan serve the function of an army for a nation with no ruler. The people of Pandaria are defended by Shado-Pan manning the Serpent's Spine, standing fast against the mantid and the yaungol, but that is the least of their duties. Their true vigil - their endless watch is against the Sha and their minions. The Shado-Pan see the hand of the Sha in the hasty march of the mantid across the Dread Wastes and Townlong Steppes and the driving of the yaungol into Kun'Lai Summit. And it is the Sha that they dedicate themselves to stopping, for they know all too well the painful effect of the Sha upon the mortal soul.
Indeed, they were created to stand against that very corrosion of the spirit.
The Shado-Pan date back to Shaohao, the last Pandaren Emperor. According to Ban Bearheart, Shaohao himself founded the order, likely during his journey to rid himself of doubt, fear, and the other negative emotions that hindered him from taking decisive action to save Pandaria from the coming disaster of the Sundering. Since this journey revealed to him the true nature of the Sha, it is likely that this revelation is what motivated him to found an order to oppose them.
Ban Bearheart says: As a stranger to these lands, you may not be familiar with the Shado-Pan.
Ban Bearheart says: Our charge is an ancient one, dating back to the time of Shaohao, our founder and the last emperor of Pandaria.
Ban Bearheart says: Dark energies embrace this land. Anger, Hatred, Fear, Doubt...
Ban Bearheart says: These negative emotions can manifest in physical form. We call this dark energy "Sha."
Ban Bearheart says: It is our sacred duty to monitor and imprison the Sha. To defeat it wherever it darkens the hearts of our people.
Ban Bearheart says: In all my years, I have never seen the Sha so active as when your soldiers landed on our shores.
Ban Bearheart says: Pandaria does not have a standing army. We, the Shado-Pan, are its first - and only - line of defense.
Ban Bearheart says: Taran Zhu leads the Shado-Pan, as his father did before him. But he has elected to close our Monastery gates - why?
Ban Bearheart says: We must find out. Without the Shado-Pan, Pandaria will surely be engulfed in darkness...
This unambiguously tells us what the first mission of the Shado-Pan is, namely to combat the Sha in all their forms. And it is also impossible to miss that the Shado-Pan were simply unprepared for the clearing of the mists that Shaohao had shrouded the land in millennia before, much less the coming of the war between the Alliance and the Horde. Simply put, the Shado-Pan are not an army, but rather a small force of disciplined, trained warriors who are expected to face the ultimate horror of the Sha and as a result are forced to endure training that winnows out all but the strongest, most determined and most able to endure. An army could stand against the mantid. An army could fight back the yaungol. But in the process, the understandable anger, hostility, rage and fear caused by the organized violence of a massed army making war would, in fact, only make things worse. Thus, the Shado-Pan recruit only those few they think might have the makings of one of their order, and the initiation into the order can and does kill many of those who undertake it. This is why their numbers are so low, because they don't dare to field the vast armies of other lands or they would directly empower their greatest enemies in the process.
This Trial of the Red Blossoms that Shado-Pan undergo exists because there can be no mistakes, and even then, even those that survive it and become Shado-Pan are not immune to the influence of the Sha. The recent near-escape of the Sha of Violence from its imprisonment within the Shado-Pan Monastery points to that, but even more telling is that Taran Zhu, head of the Order, was ultimately inhabited by one of the Sha, the Sha of Hatred. Indeed, the Shado-Pan walk a fine line in their war with the Sha, for they are the ones most often exposed to their corrupting influence. Furthermore, the Shado-Pan are often obliged to use violence to stop the Sha or those under their influence, and thus must especially guard themselves against the other negative emotions, because once you have met a Sha in battle you are marked by their foul presence forever.
"From the moment your battle began, you were known and marked by the sha. And once the sha have left their mark upon you, it will never go away. Every encounter you have with them, from this day forward, will be more difficult and more terrifying. The sha know you now. They know your minds, your weaknesses, and your fears."
Trial of the Red Blossoms
The terrible secret of the Shado-Pan is that to join the order is to sentence yourself to forever do battle with the dark heart of Pandaria itself, a battle whose best possible outcome is that you die uncorrupted by these hideous spectres of a dead god's sins and even that is unlikely, because from the very first moment you meet a Sha it has marked your soul forever. As well trained, as disciplined, as skilled and as aware as each Shado-Pan is of what the Sha are and what they can do, they are still capable of losing themselves to the darkness and becoming that which they strive to oppose. Taran Zhu, angry at what the outsiders had wrought in the Jade Forest, gave in to his hatred and allowed the Sha of Hatred to take domination of his very being, only to be saved from the Sha by the very outsiders he'd come to loathe. But at least he regained control of himself.
Suna Silentstrike shows us the true danger of combat against the Sha. Like Taran Zhu, she gave into her hatred for those that she believed her enemies, in this case the yaungol who slew her husband. Unlike Taran Zhu, she was not saved in time, and died in the process of rejecting the Sha of Hatred's hold on her.
For thousands of years, the Shado-Pan have stood at the battlements, done their best to protect their people from death and worse. They give up more than their lives. They imperil their very souls that the Sha might be contained, locked away. They are living proof that the price of their people's freedom is high indeed. In patch 5.2, the Shado-Pan will be involved in opposing the return of the mogu and their Thunder King, not only due to their role as the defenders of all Pandaria but because they know that the mogu will only strengthen the Sha, and that can never be.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.