  We present an analysis of the properties of the cluster of galaxies RXJ
1821.6+6827, or NEP 5281, at a redshift z=0.816+/-0.001. RXJ 1821.6+6827 was
discovered during the optical identification of the X-ray sources in the North
Ecliptic Pole (NEP) region of the ROSAT All-Sky Survey and it is the highest
redshift cluster of galaxies of the NEP survey. We have measured spectroscopic
redshifts for twenty cluster galaxies using the Keck-I and the
Canada-France-Hawai'i (CFH) telescopes. The value for the cluster velocity
dispersion is sigma_V=775(+182,-113) km s-1. The cluster was also observed by
XMM-Newton. Both the optical and X-ray data are presented in this paper. The
cluster has an unabsorbed X-ray flux in the 2-10 keV energy band of F(2-10
keV)=1.24(+0.16,-0.23) x 10-13 erg cm2 s-1 and a K-corrected luminosity in the
same band of L(2-10 keV)=6.32(+76,-0.73)x10^44 h_50^-2 erg s-1 (90% confidence
level). The cluster X-ray bolometric luminosity is
L(BOL,X)=1.35(+0.08,-0.21)x10^45 h_50^-2 erg s-1. The data do not allow fitting
both metal abundance and temperature at the same time. The abundance is
unconstrained and can vary in the range 0.28-1.42 Z_sun while the best fit
X-ray temperature is T=4.7(+1.2,-0.7) keV. This emission weighted X-ray
temperature is a little lower, barely within the uncertainties, than the
predicted temperature, T=6.34(+0.13,-0.35) keV, from the L_X-T_X relation of
local clusters published in the literature. The optically measured
velocitydispersion is consistent with the velocity dispersion expected from the
sigma_V-T_X relationship. We also examine the point X-ray source
RXJ1821.9+6818, or NEP 5330, located to the south east of the cluster which was
identified as a QSO at z=1.692+/-0.008 in the ROSAT NEP survey.
