House of the Night Blades
A ninja school in the Port of Auspicious Voyage, in the Hon'eth Arcade.
The master of the school (said by Ayuki to be named "Darkblade, or something pretentious like that") is in prison, serving 10 years of penal servitude for attempting to kill Cai-Wen in Of Cabbages and Coins. The Prefect of the Port of Auspicious Voyage has jurisdiction over him. He was third level at the time of his capture.
In Zen Sinker Tour, the ninjas told Cai Wen that they hope to capture him and trade him for their master's freedom. If they can't free him that way, they could probably free him with about 50 li worth of bribes.
In The Scheme of Zi Pon, the ninjas were further mocked by the party, in Cai Wen's name.
Recently, Cai Wen has pursued a policy of loudly announcing his presence in the Port of Auspicious Voyage, in order to dispel any uncertainty about whether he will be attacked.
As of Night of Gates III, the school has leveled up, which may be enough to free their master.