  Early JWST observations have uncovered a new, substantial population of red
sources that might represent a previously overlooked phase of actively growing
supermassive black holes (Kocevski et al. 2023, Matthee et al. 2023, Labbe et
al. 2023). One of the most intriguing examples is an extremely red, point-like
object that was found to be triply-imaged by the strong lensing galaxy cluster
Abell 2744 (Furtak et al. 2023), allowing an unprecedented detailed look into
this enigmatic population. Here we present deep spectroscopic JWST/NIRSpec
observations of this object, Abell2744-QSO1. The spectroscopy confirms that the
three images are of the same object, and that it is a highly reddened
($A_V\sim3$) broad emission-line Active Galactic Nucleus (AGN) at a redshift of
$z_{\mathrm{spec}}=7.0451\pm0.0005$. From the width of H$\beta$
($\mathrm{FWHM}=2800\pm250\,\frac{\mathrm{km}}{\mathrm{s}}$) we derive a black
hole mass of $M_{\mathrm{BH}}=3_{-1}^{+2}\times10^7\,\mathrm{M}_{\odot}$. We
infer a very high ratio of black hole to galaxy mass of at least 3% and
possibly as high as 100%, an order of magnitude or more than is seen in local
galaxies. The lack of strong metal lines in the spectrum together with the high
bolometric luminosity
($L_{\mathrm{bol}}=(1.1\pm0.3)\times10^{45}\,\frac{\mathrm{erg}}{\mathrm{s}}$)
suggest that we are seeing the black hole in a phase of rapid growth, accreting
at 30% of the Eddington limit. Based on early JWST imaging studies we estimate
that such heavily reddened, low-mass black holes can be $\sim100$ times more
common than UV-selected ones at this epoch. The rapid growth and high black
hole to galaxy mass ratio of A2744-QSO1 suggests that it may represent the
missing link between black hole seeds (Inayoshi et al. 2020; Greene et al.
2020; Volonteri 2021) and the first luminous quasars (Fan et al. 2023).
