  We report evidence of a fully established galaxy cluster at z=2.07,
consisting of a ~20sigma overdensity of red, compact spheroidal galaxies
spatially coinciding with extended X-ray emission detected with XMM-Newton. We
use VLT VIMOS and FORS2 spectra and deep Subaru, VLT and Spitzer imaging to
estimate the redshift of the structure from a prominent z=2.07 spectroscopic
redshift spike of emission-line galaxies, concordant with the accurate 12-band
photometric redshifts of the red galaxies. Using NICMOS and Keck AO
observations, we find that the red galaxies have elliptical morphologies and
compact cores. While they do not form a tight red sequence, their colours are
consistent with that of a >1.3$~Gyr population observed at z~2.1. From an X-ray
luminosity of .2*10^43 erg s^-1 and the stellar mass content of the red galaxy
population, we estimate a halo mass of 5.3-8*10^13 Msun, comparable to the
nearby Virgo cluster. These properties imply that this structure could be the
most distant, mature cluster known to date and that X-ray luminous,
elliptical-dominated clusters are already forming at substantially earlier
epochs than previously known.
