High pressure provides an effective route to activate molecular nitrogen and access nitrogen-rich compounds with unusual bonding motifs. Here we investigate the Eu–N system under pressure by combining attention-coupled neural network (ACNN)-assisted structure searches, first-principles calculations, and laser-heated diamond-anvil-cell experiments. Convex-hull analyses reveal a rich Eu–N phase landscape across 0–100 GPa, including nine thermodynamically stable $\mathrm{Eu}{\mathrm{N}}_x$ phases with nitrogen motifs ranging from isolated or dimeric units to cyclic, chainlike, layered, and molecularly confined configurations. Guided by these predictions, we synthesized a previously unreported $R\overline{3}m {\mathrm{EuN}}_{12}$ phase at 45–56 GPa, representing an experimental realization of this nitrogen-rich europium nitride stoichiometry. ${\mathrm{EuN}}_{12}$ contains dense lattice-confined ${\mathrm{N}}_2$-like dumbbells with a short N–N distance of 1.133 Å at 50 GPa and remains traceable upon decompression to 5 GPa, indicating the robustness of its confined molecular nitrogen units. Electronic-structure and bonding analyses suggest that localized Eu $4f$ states act as a valence-buffering reservoir, moderating Eu-to-nitrogen charge transfer and limiting occupation of ${\mathrm{N}}_2$-derived antibonding states. These results establish the Eu–N system as a rare-earth platform for nitrogen-rich compounds and reveal how pressure, localized $4f$ electrons, and nitrogen-motif evolution cooperate to stabilize high-pressure nitrides with confined molecular nitrogen units.

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• Band gap
• Crystal structure
• Density of states
• Local density of states
• First-principles calculations
• Raman spectroscopy
• X-ray diffraction