I am the PI of Arcus, a high resolution X-ray grating spectrometer mission to be proposed to NASA as a MIDEX in 2016. A mission with similar goals was previously proposed as a SMEX on ISS in 2014. Arcus will observe astrophysical phenomena over an incredibly broad range, in a sorely underutilized bandpass. Its mission includes investigations from small scales to the largest, including the composition of minute interstellar dust grains, stellar evolution, feedback from supermassive black holes, and structure formation in galaxy clusters. Using technologies developed for ESA's Athena mission and in the world-class laboratories of the University of Iowa and MIT/Lincoln Labs, Arcus will achieve resolution R (= λ/Δλ) ~ 3000 over its ~10-50 Å bandpass, an improvement of an order of magnitude over existing or approved observatories. After a short calibration period after launch in 2023, it will operate as a general observatory to which all members of the scientific community are strongly encouraged to apply.