0.08$   $0.16 \pm 0.01$  
  $f_{\rm Q}$ (red galaxies)           $0.80 \pm 0.06$   $0.67 \pm 0.11$   $0.61 \pm 0.03$  
  $f_{\rm Q}$ ($M_* \ge 10^{10}$)      $0.76 \pm 0.13$   $0.44 \pm 0.14$   $0.28 \pm 0.01$  
  $f_{\rm Q}$ ($M_* \ge 10^{10.5}$)    $0.91 \pm 0.09$   $0.38 \pm 0.16$   $0.36 \pm 0.02$  

  : Fractions of quiescent galaxies in 7C, ClG0218.3$-$0510 and in the control field UDS at $1.5<z<1.7$. $f_{\rm Q}$ gives the quiescent galaxy fraction and $f_{\rm red}$ gives the fraction of galaxies with red colours in each sample.\
  []{data-label="table:passivefraction"}

Conclusions {#sec:conclusion}
===========

We present the first robust spectroscopic redshift of the high redshift RLAGN C, placing it at $z=1.58$. We show this radio galaxy is located in an $8.9\sigma$ galaxy overdensity, implying that it is embedded in a high redshift galaxy cluster. The cluster core contains $28\pm6$ excess galaxies brighter than $J=23.6$mag. This galaxy richness implies a cluster mass of at least several $\times10^{13}$M$_{\odot}$. Of these excess galaxies, $66\pm13$% have red colours and lie on a sequence in colour-magnitude space. The rest-frame $UBJ$ colours of these galaxies show that $80$% of the red galaxies are quiescent, therefore this is a mature cluster with a predominantly old stellar population. More than 80% of the galaxies with masses 