Messengers of the Black Spot
Ninja clan in the Harbor of Fortunate Arrival. They specialize in infiltration - leaving notes, death threats, horse heads, things like that.
Autumn Rose sent a message to the party via this house; Min Feng subsequently broke into their house and stole 5 li, a ceremonial dagger, a ninja suit, and a poem of the house's Grandmaster. She left a note in the dagger's place reading "This is my application for employment and/or training."