  We present the VIMOS Spectroscopic Survey of a Supercluster in the COSMOS
field (VIS3COS) at z~0.84. We use VIMOS high-resolution spectra (GG475 filter)
to spectroscopically select 490 galaxies in and around the super-structure and
an additional 481 galaxies in the line of sight. We present the redshift
distribution, the catalog to be made public and the first results on the
properties of individual galaxies and stacked spectra
(3500{\AA}<\lambda<4200{\AA} rest-frame). We probe a wide range of densities
and environments (from low-density field to clusters and rich groups). We find
a decrease in the median star formation rate from low to high density
environments in all bins of stellar mass and a sharp rise of the quenched
fraction (from ~10% to ~40-60%) of intermediate stellar mass galaxies
$(10<\log_{10}\left(M_\star/\mathrm{M_\odot}\right)<10.75)$ from filaments to
clusters. The quenched fraction for massive galaxies shows little dependence on
the environment being constant at ~30-40%. We find a break in the strength of
the [OII] emission, with nearly constant line equivalent widths at lower
densities (~-11{\AA}) and then a drop to ~-2.5{\AA} towards higher densities.
The break in the [OII] line strength happens at similar densities
($\log_{10}(1+\delta)\sim0.0-0.5$) as the observed rise in the quenched
fraction. Our results may provide further clues regarding the different
environmental processes affecting galaxies with different stellar masses and
highlight the advantages of a single dataset in the COSMOS field probing a wide
range of stellar masses and environments. We hypothesize that quenching
mechanisms are enhanced in high-density regions.
