  We present deep Chandra and XMM obervations of the galaxy cluster
RDCS1252.9-2927, which was selected from the ROSAT Deep Cluster Survey (RDCS)
and confirmed by extensive spectroscopy with the VLT at redshift z=1.237. With
the Chandra data, the X-ray emission from the intra-cluster medium is well
resolved and traced out to 500 kpc, thus allowing a measurement of the physical
properties of the gas with unprecedented accuracy at this redshift. We detect a
clear 6.7 keV Iron K line in the Chandra spectrum providing a redshift within
1% of the spectroscopic one. By augmenting our spectroscopic analysis with the
XMM data (MOS detectors only), we significantly narrow down the 1 sigma error
bar to 10% for the temperature and 30% for the metallicity, with best fit
values kT = 6.0(+0.7,-0.5) keV, Z = 0.36(+0.12,-0.10) Z_sun. In the likely
hypothesis of hydrostatic equilibrium, we measure a total mass of M_{500} =
(1.9+-0.3)10^14 h_70^{-1}M_sun within R_{Delta=500} = 536 kpc. Overall, these
observations imply that RDCS1252.9-2927 is the most X-ray luminous and likely
the most massive bona-fide cluster discovered to date at z>1. When combined
with current samples of distant clusters, these data lend further support to a
mild evolution of the cluster scaling relations, as well the metallicity of the
intra-cluster gas. Inspection of the cluster mass function in the current
cosmological concordance model (h,Omega_m,Omega_Lambda)=(0.7,0.3,0.7) and
sigma_8=0.7-0.8 shows that RDCS1252.9-2927 is an M* cluster at z=1.24, in
keeping with number density expectations in the RDCS survey volume.
