Hydrogen embrittlement, the degradation of mechanical behaviors due to the existence of hydrogen, is an industrially and environmentally critical problem in metals and alloys. Yet the fundamental mechanism(s) of embrittlement are still controversial, and the molecular-level damage events shrouded in mystery. In hydrogen embrittlement phenomena, the molecular-level agents of damage are hypothesized to be hydrogen-vacancy complex (Va+nH→VaH_n), hereupon called hydrogenated vacancy. Contrary to vacancy, hydrogen-vacancy complex has good thermal stability and low diffusivity. When metals undergo plastic deformation at low homologous temperature in the presence of hydrogen, the mechanically driven out-of-equilibrium dislocation processes produce extremely high concentrations of hydrogen-vacancy complexes. Under such high concentrations, these complexes prefer to grow by absorbing additional vacancies and act as the embryos for the formation of proto nano-voids. Our work provides the insight on the microscopic mechanism of hydrogen embrittlement. Moreover, this work also helps understanding some unique mechanical behaviors induced by hydrogen.