"A Swarm of Butterflies" 「胡蝶乱舞 Kochō ranbu」 is the thirteenth episode of Basilisk.
Hyōma wonders what Gennosuke action would now be, as he can't proceed towards the shogun like that, but Gennosuke explains the who purpose of this journey was to confirm Ieyasu's intention and that they can't return until they have accomplished that. He tells them, they will continue the moment Saemon returns. Nenki informs Hotarubi that he had slain Gennosuke, who wasn't able to use his dojutsu and he also easily dealt with Hyōma, since he was a blind ninja. He then found Kagerō distracted by Hotarubi's butterflies and dealt with her too, but Saemon managed to escape him. He then tells Hotarubi and Kagerō had confessed that Saemon was the one who killed Yashamaru. That makes Hotarubi angry and she tells him, he should have made sure he kills Saemon above all. But then thinks this may be divine providence as she can slay Saemon herself. Nenki wonders if she would be as Saemon can take any face, but Hotarubi tells him she won't be deceived by him. no matter whom Saemon turns into. As she holds Nenki's hand, she notices the bandage on his finger and realizes its the one she put when her snake bit "Yashamaru", who was Saemon in disguise. Saemon reveals himself, congratulating her for seeing through him. Saemon attacks Hotarubi, who tries to use her technique, but Saemon cuts her hands and stabs her in the stomach. As her consciousness starts to fade away, she sees Yashamaru and starts calling him name and tell falls of the cliff.
In the morning, Jingorō isn't happy that they would be taking a boat as he fears falling into the salty ocean water. Akeginu wonders about Hotarubi and Nenki, but Tenzen explains they can't wait for people who had not shown up yet. As the boat leaves, Oboro recalls the letter from Gennosuke and that he counts her as one of the remaining Iga enemies, so she believes he must think that she let him into a trap. Tenzen informs Akeginu that he wants to speak with Oboro alone and she wonders for what exactly. He explains that when they leave the boat, they may get attacked by Kōga and he wants to be sure Oboro is on their side no matter what. Akeginu wonders what will happen in Oboro doesn't listen to him, but he assures her, Oboro would listen to him. Tenzen goes to Oboro and tells her there is something he needs to tell her.