The realization of the full potential of nitrogen-containing solid-state materials is limited by the inert and gaseous nature of N-2. In this Communication, we describe the simple synthesis yet complex structure of the new phase Mn39Si9Nx (x = 0.84). The formation of this intermetallic subnitride appears to be facilitated by the high solubility of nitrogen in manganese metal, while its structural features are guided by the complementary internal packing strains of Mn-Si and Mn-N domains, an effect known as epitaxial stabilization. These domains intergrow into a composite structure based on the interpenetration of tetrahedrally close-packed (TCP) and Mackay cluster-like modules. We anticipate that other systems combining nitrogen with the TCP packing of metals will be similarly driven toward intergrowth, opening a path to a broader family of intermetallic nitrides.