  Finding a sample of the most massive clusters with redshifts $z>0.6$ can
provide an interesting consistency check of the $\Lambda$ cold dark matter
($\Lambda$CDM) model. Here we present results from our search for clusters with
$0.6\lesssim z\lesssim1.0$ where the initial candidates were selected by
cross-correlating the RASS faint and bright source catalogues with red galaxies
from the Sloan Digital Sky Survey DR8. Our survey thus covers
$\approx10,000\,\rm{deg^2}$, much larger than previous studies of this kind.
Deeper follow-up observations in three bands using the William Herschel
Telescope and the Large Binocular Telescope were performed to confirm the
candidates, resulting in a sample of 44 clusters for which we present
richnesses and red sequence redshifts, as well as spectroscopic redshifts for a
subset. At least two of the clusters in our sample are comparable in richness
to RCS2-$J$232727.7$-$020437, one of the richest systems discovered to date. We
also obtained new observations with the Combined Array for Research in
Millimeter Astronomy for a subsample of 21 clusters. For 11 of those we detect
the Sunyaev-Zel'dovich effect signature. The Sunyaev-Zel'dovich signal allows
us to estimate $M_{200}$ and check for tension with the cosmological standard
model. We find no tension between our cluster masses and the $\Lambda$CDM
model.
