Using ab initio evolutionary methodology for crystal structure prediction, a twofold coordinated ground-state hexagonal structure (P6/mmm, 1 f.u./cell) with peculiar double N=N bonded N-2 units was uncovered for MN2 (M = OS, Ir, Ru, and Rh) compounds. This structure exhibits an unusual incompressibility along the c axis, higher than that of diamond. At much higher pressures, an eightfold coordinated tetragonal structure (P4/mbm, 2 f.u./cell) containing MN8 cuboids was unraveled and possesses also orientational ultra-incompressibility. A unified phase transition diagram for these nitrides was thus derived to reveal the intriguing chemistry of nitrogen at extreme conditions. Formation energy calculations demonstrate that the ground state phase is synthesizable at low pressure (similar to 40 GPa) while the eightfold phase can be achieved through the phase transformation via the marcasite structure.