Clusters of galaxies at redshifts nearing one are of special importance since they may be caught at the epoch of formation .
At these high redshifts there are very few known clusters .
We present follow-up ASCA , ROSAT HRI and Keck LRIS observations of the cluster RXJ 1716.6 + 6708 which was discovered during the optical identification of X-ray sources in the North Ecliptic Pole region of the ROSAT All-Sky Survey .
At z = 0.809 , RXJ 1716.6 + 6708 is the second most distant X-ray selected cluster so far published and the only one with a large number of spectroscopically determined cluster member velocities .
The optical morphology of RXJ 1716.6 + 6708 resembles an inverted S-shape filament with the X-rays coming from the midpoint of the filament .
The X-ray contours have an elongated shape that roughly coincides with the weak lensing contours .
The cluster has a low temperature , kT = 5.66 ^ { +1.37 } _ { -0.58 } keV , and a very high velocity dispersion \sigma _ { los } = 1522 ^ { +215 } _ { -150 } km s ^ { -1 } .
While the temperature is commensurate with its X-ray luminosity of ( 8.19 \pm 0.43 ) \times 10 ^ { 44 } h _ { 50 } ^ { -2 } erg s ^ { -1 } ( 2 - 10 keV rest frame ) , its velocity dispersion is much higher than expected from the \sigma - T _ { X } relationship of present-day clusters with comparable X-ray luminosity .
RXJ 1716.6 + 6708 could be an example of a protocluster , where matter is flowing along filaments and the X-ray flux is maximum at the impact point of the colliding streams of matter .