  We aim to reveal the nature of the reddest known substellar companion HD
206893 B by studying its near-infrared colors and spectral morphology and by
investigating its orbital motion. We fit atmospheric models for giant planets
and brown dwarfs and perform spectral retrievals with petitRADTRANS and ATMO on
the observed GRAVITY, SPHERE, and GPI spectra of HD 206893 B. To recover its
unusual spectral features, we include additional extinction by high-altitude
dust clouds made of enstatite grains in the atmospheric model fits. We also
infer the orbital parameters of HD 206893 B by combining the $\sim
100~\mu\text{as}$ precision astrometry from GRAVITY with data from the
literature and constrain the mass and position of HD 206893 C based on the Gaia
proper motion anomaly of the system. The extremely red color and the very
shallow $1.4~\mu\text{m}$ water absorption feature of HD 206893 B can be fit
well with the adapted atmospheric models and spectral retrievals. Altogether,
our analysis suggests an age of $\sim 3$-$300~\text{Myr}$ and a mass of $\sim
5$-$30~\text{M}_\text{Jup}$ for HD 206893 B, which is consistent with previous
estimates but extends the parameter space to younger and lower-mass objects.
The GRAVITY astrometry points to an eccentric orbit ($e =
0.29^{+0.06}_{-0.11}$) with a mutual inclination of $< 34.4~\text{deg}$ with
respect to the debris disk of the system. While HD 206893 B could in principle
be a planetary-mass companion, this possibility hinges on the unknown influence
of the inner companion on the mass estimate of
$10^{+5}_{-4}~\text{M}_\text{Jup}$ from radial velocity and Gaia as well as a
relatively small but significant Argus moving group membership probability of
$\sim 61\%$. However, we find that if the mass of HD 206893 B is $<
30~\text{M}_\text{Jup}$, then the inner companion HD 206893 C should have a
mass between $\sim 8$-$15~\text{M}_\text{Jup}$.
