  Observations of quasars reveal that many supermassive black holes (BHs) were
in place less than 700 million years after the Big Bang. However, the origin of
the first BHs remains a mystery. Seeds of the first BHs are postulated to be
either light (i.e., $10-100~\rm{M_{\odot}})$, remnants of the first stars or
heavy (i.e., $10^4-10^5~\rm{M_{\odot}})$, originating from the direct collapse
of gas clouds. Harnessing recent data from the Chandra X-ray Observatory, we
report the detection of an X-ray-luminous massive BH in a
gravitationally-lensed galaxy identified by JWST at $z\approx10.3$ behind the
cluster lens Abell 2744. This heavily-obscured quasar with a bolometric
luminosity of $L_{\rm bol}\sim5\times10^{45}~\rm{erg\ s^{-1}}$ harbors a
$M_{\rm BH}\sim10^7-10^8~\rm{M_{\odot}}$ BH assuming accretion at the Eddington
limit. This mass is comparable to the inferred stellar mass of its host galaxy,
in contrast to what is found in the local Universe wherein the BH mass is
$\sim0.1\%$ of the host galaxy's stellar mass. The combination of such a high
BH mass and large BH-to-galaxy stellar mass ratio just $\sim$500 Myrs after the
Big Bang was theoretically predicted and is consistent with a picture wherein
BHs originated from heavy seeds.
