Looking for a story

Can anyone help me find this story? Some of the characters have a special implanted organ which allows them to shapeshift and modify their bodies. There's conspiracy, action, revenge. At one point the main character becomes a medusa-like tall creature with skin like stone. At yet another chapter the main character gets stuck as a mermaid-like creature. The main character starts out plotting revenge, becomes a villain, then becomes good again. I've been trying out various keywords but just can't find it. Thanks in advance!