We propose to obtain spatially resolved X-ray spectroscopy of a sample of 20 massive (M>5e+14M_sun) clusters detected blindly by Planck in the red
shift range 0.7<z<0.9. Using for the first time a statistically significant sample in this extreme-mass, high-redshift regime, we will extend our study of
the pressure, entropy and mass profiles, their dispersion, and the fundamental scalings between YSZ, YX and M500, by more than a Gyr of lookback time. The sample constitutes an important probe of the physics of gravitational collapse on cluster scales, and will be of large legacy value for the cosmological
exploitation of the Planck sample.