  We present a large X-ray selected serendipitous cluster survey based on a
novel joint analysis of archival Chandra and XMM-Newton data. The survey
provides enough depth to reach clusters of flux of $\approx 10^{-14} {ergs}
{cm}^{-2} {s}^{-1}$ near $z$ $\approx$ 1 and simultaneously a large enough
sample to find evidence for the strong evolution of clusters expected from
structure formation theory. We detected a total of 723 clusters of which 462
are newly discovered clusters with greater than 6$\sigma$ significance. In
addition, we also detect and measure 261 previously-known clusters and groups
that can be used to calibrate the survey. The survey exploits a technique which
combines the exquisite Chandra imaging quality with the high throughput of the
XMM-Newton telescopes using overlapping survey regions. A large fraction of the
contamination from AGN point sources is mitigated by using this technique. This
results in a higher sensitivity for finding clusters of galaxies with
relatively few photons and a large part of our survey has a flux sensitivity
between $10^{-14}$ and $10^{-15} {ergs} {cm}^{-2} {s}^{-1}$. The survey covers
41.2 square degrees of overlapping Chandra and XMM-Newton fields and 122.2
square degrees of non-overlapping Chandra data. We measure the log N-log S
distribution and fit it with a redshift-dependent model characterized by a
luminosity distribution proportional to $e^{-\frac{z}{z_0}}$. We find that
$z_0$ to be in the range 0.7 to 1.3, indicative of rapid cluster evolution, as
expected for cosmic structure formation using parameters appropriate to the
concordance cosmological model.
