In this work we have proposed original superhard phases of the B-C-N system. The approach was based on crystal chemistry rationale starting from seed BCT C 4 used to develop larger C 16 cells with lower symmetry and 6 different Wyckoff positions allowing for different substitutions by B and N leading to B 8 N 8 boron nitride, and by creating defects to get C 6 N 8 carbon nitride, all three phases being isoelectronic with diamond. All three allotropes were found mechanically (elastic constants) and dynamically (phonons) stable. Furthermore, their electronic band structures are characterized by large band gaps (~5 eV) alike diamond. The hardness assessment based on three contemporary models evidences that C 16 is ultra-hard like diamond whereas two binary compounds are superhard.