  Deep observations with JWST have revealed an emerging population of red
point-like sources that could provide a link between the postulated
supermassive black hole seeds and observed quasars. In this work we present a
JWST/NIRSpec spectrum from the JWST Cycle 1 UNCOVER Treasury survey, of a
massive accreting black hole at $z=8.50$, displaying a clear broad-line
component as inferred from the H$\beta$ line with FWHM = $3439\pm413$ km
s$^{-1}$, typical of the broad line region of an active galactic nucleus (AGN).
The AGN nature of this object is further supported by high ionization, as
inferred from emission lines, and a point-source morphology. We compute the
black hole mass of log$_{10}(M_{\rm BH}/M_\odot)=8.17\pm0.42$, and a bolometric
luminosity of $L_{\rm bol}\sim6.6\times10^{45}$ erg s$^{-1}$. These values
imply that our object is accreting at $\sim 40\%$ of the Eddington limit.
Detailed modeling of the spectral energy distribution in the optical and
near-infrared, together with constraints from ALMA, indicate an upper limit on
the stellar mass of log$_{10}(M_{\rm *}/M_\odot)<8.7$, which would lead to an
unprecedented ratio of black hole to host mass of at least $\sim 30 \%$. This
is orders of magnitude higher compared to the local QSOs, but is consistent
with recent AGN studies at high redshift with JWST. This finding suggests that
a non-negligible fraction of supermassive black holes either started out from
massive seeds and/or grew at a super-Eddington rate at high redshift. Given the
predicted number densities of high-$z$ faint AGN, future NIRSpec observations
of larger samples will allow us to further investigate the galaxy-black hole
co-evolution in the early Universe.
