  Brown dwarfs with well-measured masses, ages and luminosities provide direct
benchmark tests of substellar formation and evolutionary models. We report the
first results from a direct imaging survey aiming to find and characterize
substellar companions to nearby accelerating stars with the assistance of the
Hipparcos-Gaia Catalog of Accelerations (HGCA). In this paper, we present a
joint high-contrast imaging and astrometric discovery of a substellar companion
to HD 176535 A, a K3.5V main-sequence star aged approximately
$3.59_{-1.15}^{+0.87}$ Gyrs at a distance of $36.99 \pm 0.03$ pc. In advance of
our high-contrast imaging observations, we combined precision HARPS RVs and
HGCA astrometry to predict the potential companion's location and mass. We
thereafter acquired two nights of KeckAO/NIRC2 direct imaging observations in
the $L'$ band, which revealed a companion with a contrast of $\Delta L'_p =
9.20\pm0.06$ mag at a projected separation of $\approx$0.$\!\!''35$
($\approx$13 AU) from the host star. We revise our orbital fit by incorporating
our dual-epoch relative astrometry using the open-source MCMC orbit fitting
code $\tt orvara$. HD 176535 B is a new benchmark dwarf useful for constraining
the evolutionary and atmospheric models of high-mass brown dwarfs. We found a
luminosity of $\rm log(L_{bol}/L_{\odot}) = -5.26\pm0.07$ and a model-dependent
effective temperature of $980 \pm 35$ K for HD 176535 B. Our dynamical mass
suggests that some substellar evolutionary models may be underestimating
luminosity for high-mass T dwarfs. Given its angular separation and luminosity,
HD 176535 B would make a promising candidate for Aperture Masking
Interferometry with JWST and GRAVITY/KPIC, and further spectroscopic
characterization with instruments like the CHARIS/SCExAO/Subaru integral field
spectrograph.
