We demonstrate a high-throughput, high-damage-threshold beam separator for wavelengths shorter than $30\mathrm{nm}$ , which uses a $10\mathrm{nm}$ thick niobium nitrogen film prepared on a Si substrate, set at the Brewster angle relative to the pump wavelength. The film was deposited by rf reactive magnetron sputtering on a Si substrate. The beam separator has an attenuation ratio of 0.01 and a damage-threshold intensity of at least $0.8\mathrm{TW}∕{\mathrm{cm}}^2$ for a $26\mathrm{fs}$ pump pulse. The measured reflectivity of the beam separator exceeded 70% in a wavelength range of $13–18\mathrm{nm}$ . This broadband beam separator may be used to eliminate energetic laser pulses from longitudinally pumped x ray lasers as well as from high-order harmonics.