The simultaneous realization of intrinsic magnetism and superhardness within a single light-element lattice represents a fundamental paradox, as the strong ${sp}^3$ covalency required for rigidity inherently suppresses the localized moments essential for magnetic ordering. Here, we resolve this dichotomy through the theoretical design of metastable 3D ${\mathrm{C}}_7{\mathrm{N}}_1$ covalent networks, constructed via a stacking engineering strategy that couples graphene with two-dimensional ${\mathrm{C}}_3{\mathrm{N}}_1$. We demonstrate that the coordination frustration imposed by nitrogen induces stable magnetic moments on threefold coordinated carbon vertices within a superhard scaffold. Crucially, we uncover a universal magnetostructural correlation: an increase in out-of-plane buckling consistently enhances the local magnetic moment. This coupling is mechanically driven by the differential compressibility between compliant interlayer C–C pillars and stiff steric N–C repulsions, and electronically governed by pyramidalization-induced localization, which narrows the defect bandwidth to satisfy the Stoner criterion. This architecture preserves exceptional mechanical integrity, yielding a Vickers hardness of 59 GPa and a Debye temperature of 1894 K. Electronic structure analysis utilizing the HSE06 hybrid functional reveals that both allotropes are intrinsic wide-gap semiconductors, exhibiting fundamental band gaps of 1.47 and 1.65 eV for the AA- and ABC-stacked phases, respectively. Beyond thermodynamics, we identify a kinetically viable synthesis pathway driven by nonhydrostatic mechanics, where shear stress significantly lowers the phase transformation barrier to an accessible threshold of $\sim 32\mathrm{GPa}$. Furthermore, a giant magnetostructural coupling is observed, where external pressure drives a reversible Stoner transition from a magnetic semiconductor to a nonmagnetic metal. This work establishes ${\mathrm{C}}_7{\mathrm{N}}_1$ as a versatile platform for exploring correlated electron physics within superhard architectures.

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• Hardness
• Magnetism
• Pressure effects
• Spintronics
• First-principles calculations