Tycho Nestoris begins by asking who the Faceless Men are. He answers that it depends on who is asking. To a penetant, they may be relief. To a victim, they may be vengeance. To a lord, they may be an incredible expense and to the Iron Bank they may be just another asset.
Nobody but the Faceless Men know their origin, but the order is rumored to predate the founding of Braavos. It is only know that they reside in, or are somehow alligned with, the House of Black and White, an odd and lonely building where few who enter ever leave. Some whisper that those weirwood doors open not on a house, but onto the world of the dead, from whence the Faceless Men rise when summoned.
Tycho states that such foolishness and can dismissed as easily as the fishwives who spread it. He insists that the House of Black and White is merely a temple consecrated to the Many-Faced God and filled with statuary of his many faces: The Old Gods of Westeros, the Lord of Light, the Black Goat, the Lion of Night, the Weeping Lady, The Stranger. Perhaps now it can be understood what is worshipped there, and along with them other gods whom none alive recognise, brought there long ago by sailors who never came again.
Unlike the priests of other religions, the servants there preach no sermons and perform no ceremonies. As far as one can see, silence and solitude for the whole of their worship, as well as collecting the occasional...devotee. If one wants to engage a Faceless Man, one visits the House of Black and White and pays the price. As for what that may be, those who have paid it rarely speak of it. For make no mistake, from the moment the Faceless Men accept your offer, the man you named is dead, though he does not know it. Perhaps no that day, perhaps not that year, but soon and inevitably. Many would say that the person that paid is as much his murderer as if they had swung the axe themselves. Just as many would say that is the point.