We have built and commissioned a novel X-ray spectrometer (MOSARIX) in the von Hamos configuration based on highly annealed pyrolytic graphite (HAPG) crystals. The spectrometer is optimized for the energy range of 2 to 5 keV, but this range can be extended up to 20 keV by using a higher reflection order. With its nine crystals and a Pilatus detector, MOSARIX achieves unprecedented detection efficiency with good energy resolution (> 4000), opening the door to study small cross-section phenomena and to perform fast in-situ measurements; for instance, at the sulfur edge which is of great interest for battery development. The spectrometer operates under a He atmosphere, which provides a flexible sample environment for measurements in gas, liquid and solid phases. We will present this new instrument with the first scientific results obtained recently. RIXS maps were measured at the sulfur edge for several sulfur-containing molecules. In addition, clear evidence for the formation of double-core-hole states in sulfur was observed by measuring the hypersatellite Kalpha emission at the GALAXIES beamline of the SOLEIL synchrotron facility. This instrument is already in operation at the GALAXIES beamline and will be opened to external users in collaboration with our institute.