unofficial sequel
followup to a creative work that is created without the consent or involvement of the original work's creator or producer
en-US/ˌʌnəˈfɪʃəl ˈsikwəl/
en-GB/ˌʌnəˈfɪʃl ˈsiːkwəl/
followup to a creative work that is created without the consent or involvement of the original work's creator or producer