  We present band 6 ALMA observations of a heavily-obscured radio-loud
($L_{1.4\ \mathrm{GHz}}=10^{25.4}$ W Hz$^{-1}$) AGN candidate at
$z_\mathrm{phot}=6.83\pm0.06$ found in the 1.5 deg$^2$ COSMOS field. The ALMA
data reveal detections of exceptionally strong [CII]158$\mu$m
($z_\mathrm{[CII]}=6.8532$) and underlying dust continuum emission from this
object (COS-87259), where the [CII] line luminosity, line width, and 158$\mu$m
continuum luminosity are comparable to that seen from $z\sim7$ sub-mm galaxies
and quasar hosts. The 158$\mu$m continuum detection suggests a total infrared
luminosity of $9\times10^{12}$ $L_\odot$ with corresponding very large obscured
star formation rate (1300 $M_\odot$/yr) and dust mass ($2\times10^9$
$M_\odot$). The strong break seen between the VIRCam and IRAC photometry
perhaps suggests that COS-87259 is an extremely massive reionization era galaxy
with $M_\ast\approx1.7\times10^{11}$ $M_\odot$. Moreover, the MIPS, PACS, and
SPIRE detections imply that this object harbors an AGN that is heavily obscured
($\tau_{_{\mathrm{9.7\mu m}}}=2.3$) with a bolometric luminosity of
approximately $5\times10^{13}$ $L_\odot$. Such a very high AGN luminosity
suggests this object is powered by an $\approx$1.6 $\times$ 10$^9$ $M_\odot$
black hole if accreting near the Eddington limit, and is effectively a
highly-obscured version of an extremely UV-luminous ($M_{1450}\approx-27.3$)
$z\sim7$ quasar. Notably, these $z\sim7$ quasars are an exceedingly rare
population ($\sim$0.001 deg$^{-2}$) while COS-87259 was identified over a
relatively small field. Future very wide-area surveys with, e.g., Roman and
Euclid have the potential to identify many more extremely red yet UV-bright
$z\gtrsim7$ objects similar to COS-87259, providing richer insight into the
occurrence of intense obscured star formation and supermassive black hole
growth among this population.
