  The James Webb Space Telescope is now detecting early black holes (BHs) as
they transition from "seeds" to supermassive BHs. Recently Bogdan et al. (2023)
reported the detection of an X-ray luminous supermassive BH, UHZ-1, with a
photometric redshift at $z > 10$. Such an extreme source at this very high
redshift provides new insights on seeding and growth models for BHs given the
short time available for formation and growth. Harnessing the exquisite
sensitivity of JWST/NIRSpec, here we report the spectroscopic confirmation of
UHZ-1 at $z = 10.073 \pm 0.002$. We find that the NIRSpec/Prism spectrum is
typical of recently discovered z~10 galaxies, characterized primarily by
star-formation features. We see no clear evidence of the powerful X-ray source
in the rest-frame UV/optical spectrum, which may suggest heavy obscuration of
the central BH, in line with the Compton-thick column density measured in the
X-rays. We perform a stellar population fit simultaneously to the new NIRSpec
spectroscopy and previously available photometry. The fit yields a stellar mass
estimate for the host galaxy that is significantly better constrained than
prior photometric estimates ($M_*\sim 1.4^{+0.3}_{-0.4} \times 10^8 M_\odot$).
Given the predicted BH mass ($M_{\rm BH}\sim10^7-10^8 M_\odot$), the resulting
ratio of $M_{\rm BH}/M_*$ remains two to three orders of magnitude higher than
local values, thus lending support to the heavy seeding channel for the
formation of supermassive BHs within the first billion years of cosmic
evolution.
