  We report on deep SCUBA-2 observations at 850$\mu$m and NOEMA spectroscopic
measurements at 2 mm of the environment surrounding the luminous, massive
($M_{*} \approx 2 \times 10^{11}$ M$_{\odot}$) Herschel-selected source
HerBS-70. This source was revealed by previous NOEMA observations to be a
binary system of dusty star-forming galaxies at $z= 2.3$, with the East
component (HerBS-70E) hosting an Active Galactic Nucleus (AGN). The SCUBA-2
observations detected, in addition to the binary system, twenty-one sources at
$> 3.5 \sigma$ over an area of $\sim 25$ square comoving Mpc with a sensitivity
of $\sigma_{850} = 0.75$ mJy. The surface density of continuum sources around
HerBS-70 is three times higher than for field galaxies. The NOEMA spectroscopic
measurements confirm the protocluster membership of three of the nine brightest
sources through their CO(4 - 3) line emission, yielding a volume density 36
times higher than for field galaxies. All five confirmed sub-mm galaxies in the
HerBS-70 system have relatively short gas depletion times ($80 - 500$ Myr),
indicating the onset of quenching for this protocluster core due to the
depletion of gas. The dark matter halo mass of the HerBS-70 system is estimated
around $5 \times{} 10^{13}$ M$_{\odot}$, with a projected current-day mass of
$10^{15}$ M$_{\odot}$, similar to the local Virgo and Coma clusters. These
observations support the claim that DSFGs, in particular the ones with observed
multiplicity, can trace cosmic overdensities.
