  In conventional Hermitian systems with the open boundary condition, Bloch's
theorem is perturbatively broken down, which means although the crystal
momentum is not a good quantum number, the eigenstates are the superposition of
several extended Bloch waves. In this paper, we show that Bloch's theorem can
be non-perturbatively broken down in some Hermitian Bosonic systems. The
quasiparticles of the system are the superposition of localized non-Bloch
waves, which are characterized by the complex momentum whose imaginary part
determines the localization properties. Our work is a Hermitian generalization
of the non-Hermitian skin effect, although they share the same mechanism.
