Donten ni Warau Gaiden: Shukumei, Soutou no Fuuma
Cloudy Laugh Gaiden
Laughing Under the Clouds Gaiden
曇天に笑う<外伝> ~宿命、双頭の風魔~
Fuuma ninja clan is a secret ninja tribe that is waiting for the "Orochi" (gigantic snake) to resurrect for generations. Shirasu and his twin brother Ichiu grew up in this viscous environment and this tells how they become the leaders of the clan.
(Source: Amazon Prime Video)