Mermaid Saga

Mermaid Saga (人魚シリーズ Ningyo Shirīzu) is a series of manga graphic novels in three volumes by Rumiko Takahashi. Two of the stories from the series, Mermaid Forest and Mermaid's Scar, have been adapted as anime OVAs, and all of the tales, except one, were later produced as an anime TV series.

Plot
According to an ancient Japanese legend, mermaid flesh may grant immortality if eaten. However, there is a much greater chance that consumption will lead to death or transformation into a damned creature known as a Lost Soul (or Deformed Ones in the English dub). Mermaid Saga tells the tale of Yuta, an immortal who has been alive for five hundred years, who is wandering across Japan searching for a cure and meeting others whose lives have also been ruined by mermaid flesh in the process.