Guinea Pig 4: Mermaid in a Manhole (1988)

Guinea Pig 4: Mermaid in a Manhole 1988 / ザ・ギニーピッグ マンホールの中の人魚 1988 / Mermaid in a Manhole , Mermaid in a Sewer , Mermaid in the Bathtub , Guinea Pig Part 4 , Za Ginī Piggu: Manhōru no naka no Ningyo

An artist finds and rescues a mermaid in a sewer. He takes her home with him and she develops sores all over her body that begin to pustulate and bleed. He uses what oozes from her sores to paint her portrait. When he can no longer handle it anymore he breaks down and dismembers her body. (Source: IMDB)