  We report the detection of a 13$\sigma$ H$\alpha$ emission line from HDF850.1
at $z=5.188\pm0.001$ using the FRESCO NIRCam F444W grism observations.
Detection of H$\alpha$ in HDF850.1 is noteworthy, given its high far-IR
luminosity, substantial dust obscuration, and the historical challenges in
deriving its redshift. HDF850.1 shows a clear detection in the F444W imaging
data, distributed between a northern and southern component, mirroring that
seen in [CII] from the Plateau de Bure Interferometer. Modeling the SED of each
component separately, we find that the northern component has a higher mass,
star formation rate (SFR), and dust extinction than the southern component. The
observed H$\alpha$ emission appears to arise entirely from the less-obscured
southern component and shows a similar $\Delta$v$\sim$+130 km/s velocity offset
to that seen for [CII] relative to the source systemic redshift. Leveraging
H$\alpha$-derived redshifts from FRESCO observations, we find that HDF850.1 is
forming in one of the richest environments identified to date at $z>5$, with
100 $z=5.17-5.20$ galaxies distributed across 10 structures and a $\sim$(15
cMpc)$^3$ volume. Based on the evolution of analogous structures in
cosmological simulations, the $z=5.17-5.20$ structures seem likely to collapse
into a single $>$10$^{14}$ $M_{\odot}$ cluster by $z\sim0$. Comparing galaxy
properties forming within this overdensity with those outside, we find the
masses, SFRs, and $UV$ luminosities inside the overdensity to be clearly
higher. The prominence of H$\alpha$ line emission from HDF850.1 and other known
highly-obscured $z>5$ galaxies illustrates the potential of NIRCam-grism
programs to map both the early build-up of IR-luminous galaxies and overdense
structures.
