Birabira the Sea Beast

Dororo and Hyakkimaru come to a fishing village. Dororo pulls out a harpoon from a rock just for fun. The harpoon, however, has contained a skeleton ray-monster called Bira-Bira. Hyakkimaru dives into the sea following Bira-Bira, which has escaped from underground to the sea. After a duel, Hyakkimaru disappears. The villagers think he must be dead. So in order to calm Bira-Bira`s anger, they throw Dororo and a village girl into the sea as sacrifice. Source: tezukaosamu.net