  We report first results from a deep near infrared campaign with the Hubble
Space Telescope to obtain late-epoch images of the Hubble Ultra-Deep Field
(HUDF), 10-15 years after the first epoch data were obtained. The main
objectives are to search for faint active galactic nuclei (AGN) at high
redshifts by virtue of their photometric variability, and measure (or
constrain) the comoving number density of supermassive black holes (SMBHs),
n_{SMBH}, at early times. In this Letter we present an overview of the program
and preliminary results concerning eight objects. Three variables are
supernovae, two of which are apparently hostless with indeterminable redshifts,
although one has previously been recorded at a z\approx 6 object precisely
because of its transient nature. Two further objects are clear AGN at z= 2.0
and 3.2, based on morphology and/or infrared spectroscopy from JWST. Three
variable targets are identified at z = 6-7, which are also likely AGN
candidates. These sources provide a first measure of n_{SMBH} in the
reionization epoch by photometric variability, which places a firm lower limit
of 3 \times 10^{-4} cMpc^{-3}. After accounting for variability and luminosity
incompleteness, we estimate n_{SMBH} \gtrsim 8 \times 10{-3} cMpc{-3}, which is
the largest value so far reported at these redshifts. This SMBH abundance is
also strikingly similar to estimates of n_{SMBH} in the local Universe. We
discuss how these results test various theories for SMBH formation.
