  High-redshift quasars are thought to live in the densest regions of space
which should be made evident by an overdensity of galaxies around them.
However, campaigns to identify these overdensities through the search of Lyman
Break Galaxies (LBGs) and Lyman $\alpha$ emitters (LAEs) have had mixed
results. These may be explained by either the small field of view of some of
the experiments, the broad redshift ranges targeted by LBG searches, and by the
inherent large uncertainty of quasar redshifts estimated from UV emission
lines, which makes it difficult to place the Ly-$\alpha$ emission line within a
narrowband filter. Here we present a three square degree search ($\sim 1000$
pMpc) for LAEs around the $z=6.9$ quasar VIKJ2348-3054 using the Dark Energy
CAMera (DECam), housed on the 4m Blanco telescope, finding 38 LAEs. The
systemic redshift of VIK J2348--3054 is known from ALMA [CII] observations and
place the Ly-$\alpha$ emission line of companions within the NB964 narrowband
of DECam. This is the largest field of view LAE search around a $z>6$ quasar
conducted to date. We find that this field is $\sim$ 10 times more overdense
when compared to the Chandra Deep-Field South, observed previously with the
same instrumental setup as well as several combined blank fields. This is
strong evidence that VIKJ2348-3054 resides in an overdensity of LAEs over
several Mpc. Surprisingly, we find a lack of LAEs within 5 physical Mpc of the
quasar and take this to most likely be evidence of the quasar suppressing star
formation in its immediate vicinity. This result highlights the importance of
performing overdensity searches over large areas to properly assess the density
of those regions of the Universe.
