We study the phase relations and mineral chemistry in the systems muscovite–NH3–N2-H2O and eclogite + muscovite–NH3–N2-H2O at 6.3–7.8 GPa, 1000–1200 °C, and oxygen fugacity (fO2) from ∼IW (Fe–FeO) to ∼ NNO (Ni–NiO) equilibria. The quenched H2O-bearing fluids differ in nitrogen speciation from NH3-rich to N2–rich, and the respective N2/(NH3+N2) ratio varies from <0.1 to ∼ 1. N-bearing K-cymrite is obtained in association with a kyanite-garnet-jadeite ± muscovite ± corundum assemblage in the muscovite–NH3–N2-H2O system and coexists with pyrope-almandine garnet and omphacite in the eclogite + muscovite–NH3–N2-H2O system. The presence of an N-bearing fluid in the studied systems stabilizes the K-cymrite structure. Muscovite does not convert to K-cymrite in the absence of NH3–N2-bearing fluid up to 7.8 GPa and 1070–1120 °C. According to FTIR and Raman spectroscopy, K-cymrite in equilibrium with an N-rich fluid can capture a huge amount of nitrogen in cages of its framework, mainly as N2 molecules at fO2 ∼NNO and predominantly as NH3 molecules at fO2 ∼IW. The storage capacity of K-cymrite with respect to nitrogen increases from 2.9 to 6.3 wt% with increase of fO2. FTIR spectroscopy of muscovite equilibrated with K-cymrite shows that the clathrate mechanism of nitrogen entrapment by aluminosilicates (as neutral N2 and NH3 molecules) is much more efficient than the K+ → (NH4)+ substitution. The structure of N-bearing K-cymrite (K,(NH4 +))[AlSi3O8]·(N2,NH3,H2O) determined using X-ray single-crystal diffraction is very similar to that of H2O-bearing K- and Ba-cymrites. It includes aluminosilicate layers consisting of double six-membered tetrahedral rings and cation sites statistically occupied with K+, Ba2+ and (NH4)+ on the six-fold symmetry axis in interlayer space. The N2 and NH3 molecules are located near the cage centers and, unlike H2O molecules, are included in the coordination environment of the cations. Our study confirms that NH3- and N2-rich K-cymrite may be stable in metapelites and can act as a redox insensitive carrier of nitrogen to >250 km mantle depths in downgoing slabs. The stability field of N-rich K-cymrite in the presence of an N2–H2O–NH3-bearing fluid is inferred to be P ≥ 4 GPa in metasediments rich in K-feldspar and P ≥ 6 GPa in those containing phengite. As the slab material sinks deeper than 250–300 km where N-bearing K-cymrite may lose stability, the releasing nitrogen may migrate to metal-saturated mantle and become stored there in γ−Fe, Fe3C, metal melt, or even iron nitride phases.