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Using deep two-color near-infrared HST imaging and unbiased grismspectroscopy we present a detailed study of the z=1.803 JKCS041 cluster.Uniquely, for a high redshift cluster, we confirm a mass of $\log M=14.2$ insolar units using four different techniques based on the X-ray temperature, theX-ray luminosity and the cluster richness. JKCS041 is thus a progenitor of alocal system like the Coma cluster. Our rich dataset and the abundantpopulation of 14 spectroscopically-confirmed red sequence galaxies allows us toexplore the past star formation history of this system in unprecedented detail.Remarkably, we find a prominent red sequence down to stellar masses as low as$\log M=9.8$, corresponding to a mass range of 2 dex. These quiescent galaxiesare concentrated around the cluster center with a core radius of 330 kpc. Bluemembers are few and avoid the cluster center. In JKCS041 quenching wastherefore largely completed by a look-back time of 10 Gyr and we can constrainthe epoch at which this occurred via spectroscopic age-dating of the individualgalaxies. Most galaxies were quenched about 1.1 Gyr prior to the epoch ofobservation. The less massive quiescent galaxies are somewhat younger,corresponding to a decrease in age of 650 Myr per mass dex, but the scatter inage at fixed mass is only 380 Myr (at $\log M=11$). The size-mass relation ofquiescent galaxies in JKCS041 is consistent with that observed for localclusters within our uncertainties. Comparing our data on JKCS041 with 41clusters at lower redshift, we find that the form of the mass function of redsequence galaxies has hardly evolved in the past 10 Gyr, both in terms of itsfaint end slope and characteristic mass. Despite observing JKCS041 soon afterits quenching and the three fold expected increase in mass in the next 10 Gyr,it is already remarkably similar to present-day clusters.

JKCS041: a Coma cluster progenitor at z=1.803