We propose to obtain GMOS spectroscopy of the 20 richest galaxy clusters at z ~ 1 and above over the 18,120 square degree area covered by the Massive and Distant Clusters of WISE Survey (MaDCoWS). The goal of our spectroscopic program is to confirm the most massive clusters at high redshift in the extragalactic sky, which will form a unique legacy for the community to exploit through detailed followup using e.g. Chandra, HST, ALMA, and JWST, and for comparison with upcoming cluster samples at similar redshifts from e.g. SPT3G and eRosita. The largest virialized structures at such high redshifts will be used 1) in the f_gas and primordial non-Gaussianity cosmological tests; 2) for the unbiased calibration of the scaling relations used in cluster mass estimates; 3) for strong lensing studies of cluster concentrations and central mass distributions, as well as identifying lensed high-z targets for JWST and ALMA studies of galaxy formation and reionization; and 4) to test models of massive galaxy evolution in the most extreme environments.