Single stars end their lives in either of three objects -- white dwarfs, neutron stars, and black holes; eventually "dark" objects. When they form in a pair, they produce one of the most spectacular astrophysical events and hence laboratories of extreme physical conditions. In this talk, I will present a general overview of the astrophysical events following the merger of these compact stars at the end state of their evolution in a binary system in light of my own research. More specifically, I will confine to Type Ia supernovae from binary white dwarf mergers, kilonovae, and gravitational waves from binary neutron star mergers. I will discuss what can we learn about the conditions of matter which is usually not possible by terrestrial experiments and about the universe for which these are excellent tools.
