  Models for the origin of the hard X-ray background have suggested that
sources with the most accretion activity lie hidden in highly obscured AGN. We
report on our study of hard, serendipitous sources in the fields of Chandra
clusters with fluxes close to the turn-over in the source-counts. These include
two Type II quasars with measured X-ray luminosities >10^{45} ergs per second
and column-densities >10^{23} cm^{-2}, one possibly being Compton-thick. Both
show indications of redshifted Fe K\alpha line emission. Radiative transfer
modelling of the broad-band spectrum of a highly-magnified source with deep
ISOCAM detections implies the presence of warm-to-hot dust obscuring a quasar
with L_{UV}>10^{45} ergs per second. Multi-wavelength spectroscopic and
photometric follow-up of the optically-faint sources suggests that these
objects are found in the centres of massive evolved galaxies at a range of
redshifts, with red optical / near-infrared colours dominated by the host
galaxy. Detailed source identification is difficult due to the paucity of
strong emission features, especially in the infrared. We present the main
results from a sample of near-infrared spectra of optically-faint sources
obtained with 4 m and 8 m telescopes. Through the study of the harder and
brighter X-ray background source population, we are likely to be viewing the
most intense phase of the growth of supermassive black holes.
