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Cluster environments in the local universe are dominated by passive comma quiescent galaxies while proto-clusters at z>2 typically contain large fractions of actively star-forming galaxies. The physics underlying this observed transformation of the cluster member population has been a long standing puzzle in astronomy. The cluster quenching efficiency appears to rapidly increase over cosmic time but whether this is due to the pre-processing of galaxies in group environments prior to infall or occurs during accretion into the cluster itself is V1.0. Our understanding has been limited by the lack of virialized clusters discovered at the critical transformational redshift of zvirgul2 comma until now. We propose HST observations of XLSSC 122 - the highest redshift (z=1.98) comma virialized comma massive galaxy cluster. We will utilize G141 grism observations to determine the redshifts of galaxies out to twice the cluster virial radius and combine these with WFC3 F140W-F105W and ACS F814W imaging to constrain the star-formation activity in these galaxies. These data will provide unique comma unparalleled insight into the cosmic evolution of environmental quenching and unveil its impact in the cluster outskirts and in nearby groups and filaments.