The Silverhair Knights, also called sin eaters, were an order of priests of the goddess Eilistraee, completely dedicated to the redemption and subsequent protection of drow followers of Lolth, Vhaeraun, and other deities of the Dark Seldarine. They fully embraced the teaching of Eilistraee about showing mercy and compassion, and fought dark elves using only nonlethal means, hoping to make them understand the error of their ways and choose a different life.
Silverhair Knights vowed to never willingly kill any drow and were thus specifically trained at dealing nonlethal violence. They aimed to show to other dark elves the strength that could be found in compassion, and that living in peace, without the endless conflict and paranoia that ruled their society, was a concrete possibility. They hoped that other drow would see that the Silverhair Knights honestly meant them no harm, and so would feel safer about joining them. The Silverhair Knights believed that this could lead the drow to choose to abandon the teachings of Lolth and cleave to Eilistraee.
Lethal force against another drow was only permitted when absolutely necessary, when it would directly lead to other lives being saved. Even then, a silverhair knight had to atone for her act. For the same reason, those who had caused the death of a drow in their past had to seek atonement from another cleric of the goddess in order to join the order. Atonement was usually through a quest for the church of Eilistraee.
The Silverhair Knights operated mostly in areas where drow were in the process of gaining a foothold on the surface, mostly in Cormanthor.
When they found an encampment or settlement of drow, their duty was to observe the group from afar, learning about them, and identifying those who could be turned from Lolth's ways. Eventually, they would infiltrate the group, approach possible converts, and enable their escape to a sanctuary. Generally, silverhair knights weren't active near cities or other large metropolitan areas unless they had to escort drow converts to a sanctuary within.
Silverhair Knights were mostly drow and half-drow priestesses of Eilistraee, but they could be of any race or gender. They were often clerics, but usually trained also as bards, fighters, and rangers.
As an order of the church of Eilistraee, dedicated to the redemption of the drow, the Silverhair Knights were primarily enemies of the churches of the evil deities of the Dark Seldarine (namely Lolth, Vhaeraun, Kiaransalee, and Ghaunadaur. The Silverhair Knights didn't hate their drow enemies, and pitied them because of the wicked lives that they led.
The Silverhair Knights had a wide array of tools dedicated to the redemption of the drow and to nonlethal fighting. They were specialized in wielding their swords to incapacitate opponents rather than outright killing them (and were quite efficient with said skill) and had spells capable of incapacitating targets by various means.
With the merciful edge technique, they could fight as well trying to only subdue a foe as they could trying to harm them outright. With the blade of compassion technique, they could more effectively weaken and tire an opponent without injury. However, this tactic required great focus, and the Silverhair Knight could not be distracted by a shield or second weapon.
Silverhair Knights could make their opponents feel the weight of the suffering and injuries that they had inflicted upon other creatures (like the awaken sins spell); this was more effective against drow. This could distract, weaken, or stun a villain, and even cause mental damage.
Silverhair Knights were also blessed with a voice of serenity, gaining a magically trustworthy tone and friendly lilt. This had the power to soothe suspicion and doubt, to make the speaker more persuasive and believable (even in deceit), and to enhance enchantment spells they cast.
Powerful Silverhair Knights had the ability to "consume sins", to take the full weight of cruelty and suffering inflicted by evil beings' (mostly fey, humanoids, monstrous humanoids, and giants, but especially drow) upon themselves, which gave the Silverhair Knights their nickname: sin eaters. This was a complex and dangerous ritual, taking some minutes, that required the sin eater to maintain uninterrupted physical contact with their subject. The subject creature could be willing or unwilling, usually kept bound in the latter cases, or else unaware of the sin eater's intent if the sin eater chose to disguise it. When fully performed, the target creature felt the weight of all their sins on their conscience, understanding firsthand the errors of their ways. In cases of success, the target creatures were freed from their evil, regretted their past actions, and chose a different path from evil, often taking after the Knight, while the sins themselves were absorbed into the sin eater's soul and destroyed in the light of their purity and faith. In cases of failure, the sin eaters themselves were overcome by the absorbed sins; filled with despair, grief, and rage; drained of their vitality; and fell into a coma for a full day. This could potentially kill the sin eater, but they would rise again as a ghost, with the same evil ways as the one they had tried to redeem. A sin eater could only attempt this risky ritual once a week, and only perform it on an individual sinner once a year.
The Silverhair Knights also developed a number of useful spells. They developed the awaken sins spell, after the ability they knew innately. Another was the moon bolt spell, which could project bolts of moonlight capable of temporarily draining the strength of their targets and paralyzing undead creatures, and the moon lust spell, which could cause a creature to become obsessed with the moon, distracting them. With the Eilistraee's grace spell, the knights could surround their body with the divine energy of the goddess, becoming more graceful, faster, and projecting a soft silvery light around them. It could also help their sword overcome the protection from damage that some evil creatures had.