  The next generation of X-ray telescopes have the potential to detect faint
quasars at very high redshift and probe the early growth of massive black holes
(BHs). We present modelling of the evolution of the optical and X-ray AGN
luminosity function at 2 < z < 6 based on a CDM merger-driven model for the
triggering of nuclear activity combined with a variety of fading laws. We
extrapolate the merger-driven models to z > 6 for a range of BH growth
scenarios. We predict significant numbers of sources at z ~ 6 with fluxes just
an order of magnitude below the current detection limits and thus detectable
with XEUS and Constellation-X, relatively independently of the fading law
chosen. The predicted number of sources at even higher redshift depends
sensitively on the early growth history of BHs. For passive evolution models in
which BHs grow constantly at their Eddington limit, detectable BHs may be rare
beyond z ~ 10 even with Generation-X. However, in the more probable scenario
that BH growth at z > 6 can be described by passive evolution with a small duty
cycle, or by our merger driven accretion model, then we predict that XEUS and
Generation-X will detect significant numbers of black holes out to z ~ 10 and
perhaps beyond.
