In this talk, I will discuss the possibility and signature of a first-order quark-hadron phase transition happening inside the core of dying massive stars. Such a phase transition can lead to energetic supernova explosions, which would not happen without a phase transition, and leave behind massive hybrid stars. It will also result in an unambiguous signature of neutrino burst that can be detected if close by enough, and provide an alternative r-process nucleosynthesis source in the universe.