Full disclosure warning, I actually have no idea what this game is so my knowledge of what is and what isn't a thing is limited. 

Well the order is a collection of monks, priests, nuns, and paladins all dedicated to the Lady. The origin of the lady is a mystery to everyone, even the order. Scholars outside the order believe that the lady could be Gladrasil but don't pay too much attention to the little group, there are bigger things to worry about. The order itself is split up into two halves, the faith and the warriors. The nuns and monks stay in the monastery while the priests and paladins spread out, preaching and defending the Lady's will. Outside of this there isn't much for advancement and the people in the order are happy with their place but there are special select few, three, that are chosen for greater things. 

During the right of placement, an act much like the christen baptism in which the person is held underwater for a time, there is a chance that the water will turn either red, gold, and black. These symbolize the three faces of the Lady, Mercy, Faith, and Vengeance. The vessel of mercy is almost always a woman and works to absolve those of their sins, like a priest in confession. However unlike a priest, the vessels method is a little more active, literally burning the sin from the veins of others. While painful the act does remove the sin from their bodies, but this act can only be used on a person once. If they were to fall to a life of sin again then the purge would kill them. 

The vessel of faith changes genders each generation, meaning that every time there is a new vessel it will be the opposite gender of the current vessel. The job of the vessel of faith is much like a spiritual leader, supposedly conversing with the lady and making sure that the order stays on the straight path. The vessel of faith is able to tell the intentions of any who speak to it by looking not at the body, but at the soul. 

Lastly there is the vessel of vengeance, a predominately male position. Unlike the vessels of mercy or faith who stay inside the walls of the monastery and lead the others onto the good path, the vessel of vengeance goes out into the world and punishes others for crimes against the Lady. However each vessel of vengeance is different, some take up a mantel of protection and avenges all wrongs committed against the innocent while others ignore all else unless it has anything to do with the Lady. It truly is up to each vessel of vengeance how much their judgment spreads. Unlike the mercy or faith who gain magic powers, vengeance is trained in the arts of martial war and in basic magical defense, there is little more terrifying to see than a vessel of vengeance fully enlightened.