Looking for further information on this “Black Tortoise of the North” you start by checking to see what Kasuga Nana knows about Tortoise in The City of the Rich Frog. Her face darkens as soon as you mention the subject and she’s quiet for a moment.
“You probably wouldn’t have heard.” She sighs and takes a long draw on her pipe. “The higher-ups still prefer to keep that quiet. But I was here when it all happened.”
“Several years ago, Kasuga Toshiaki, one of the samurai assigned to The City of the Rich Frog, suddenly stopped submitting reports. She failed to respond to any attempts to contact her. When they sent someone to the city to make contact with some of her known assets they found that anyone with any close knowledge of her had either gone missing as well or been found dead.
“Attempts to reestablish a network in the city have been noticeably difficult. Tortoise samurai sent there have a habit of turning up dead and potential assets are understandably reluctant to talk to anyone.
“It’s still uncertain exactly what happened. What is clear is that someone doesn’t want the Tortoise in the city and is quite effective at keeping things that way.
She knocks the ash from her pipe, packs it with another pinch of tobacco from the smoking set on her desk, and leans down to light it.
“I still do business with Rich Frog — it’s too valuable to ignore — but it’s difficult. I have to obscure everything and work through cutouts. I’ve tried to recruit Captain Yellow, but so long as she carries cargo for Shosuro Gobei, I’ve been unable to tempt her away. What little I can manage to get from the city in terms of information is therefore always second-hand, but it’s better than nothing, and that’s all I’d get if I tried to get any closer.”
A wealthy grain merchant by the name of Shohei had a reputation as someone who spoke his mind, loudly, often, and without concern. In recent times this had been to criticize those from nearly all sides: the Lion had been taken over by the usurper Matsu Tsuko and cared more about politics now than any perfumed Crane, the Unicorn felt safe behind their vast lands, running away before anyone made any questions about how loyal the half-gaijin are to begin with; and the Dragon, as ever, are too busy contemplating the deeper mysteries of their own navels to get involved. Tsuko might talk big, but what had she or any of her supporters actually done? Meanwhile Toturi was widely acknowledged as having delivered the notorious bandit leader Kyomeru to justice, riding at the head of a band of Twelve Ronin who seek nothing but justice and the defense of the humble peasants who feed us all.
People paid no mind to him. That is until he stopped talking. And then only because it happened so suddenly, his body slumped over the table in a private room of a busy and respectable inn. The servant who found him reported that he had been left alone so briefly his soup was still steaming. Nothing had been disturbed except for the slender blade of a shuriken through his throat. A white ribbon on its end. On one side the characters 天誅 (tenchuu; heaven's punishment) in red ink and on the other a simple mon of the Tortoise Clan, except in black.
Already people are speculating on the identity of this fearsome hitokiri (literally: man-slayer) they're referring to as the Black Tortoise of the North.