The progression to melanoma is still poorly understood and there's a lot of research going into it. Our current understanding is that melanoma arises, either from an existing mole called a dysplastic nevus or de novo, meaning abnormal skin. As the melanoma grows, it tends to have a horizontal growth phase, meaning it will just involve the epidermis, the top layer of the skin. Later it will have a vertical growth phase, which means it will start invading into the dermis and underlying structures.
Once the invasion has happened the melanocytes that formed the melanoma can travel through the lymphatic channels, into lymph nodes and to other parts of the body, like the liver and the brain. When this happens it's called metastatic melanoma. Metastatic melanoma is the most difficult type of melanoma to treat. Depending on where it has metastasized, the mortality rate changes anywhere from 20-50%.