  The discovery and spectroscopic confirmation of Hyperion, a
proto-supercluster at z~2.47, provides an unprecedented opportunity to study
distant galaxies in the context of their large-scale environment. We carry out
deep narrow-band imaging of a ~1*1 deg^2 region around Hyperion and select 157
Lya emitters (LAEs). The inferred LAE overdensity is delta_g~40 within an
effective volume of 30*20*15 cMpc^3, consistent with the fact that Hyperion is
composed of multiple protoclusters and will evolve into a super-cluster with a
total mass of M_tot ~1.4*10^15 M_sun at z=0. The distribution of LAEs closely
mirrors that of known spectroscopic members, tracing the protocluster cores and
extended filamentary arms connected to them, suggesting that they trace the
same large-scale structure. By cross-correlating the LAE positions with HI
tomography data, we find weak evidence that LAEs may be less abundant in the
highest HI regions, perhaps because Lya is suppressed in such regions. The
Hyperion region hosts a large population of active galactic nuclei (AGN), ~12
times more abundant than that in the field. The prevalence of AGN in
protocluster regions hints at the possibility that they may be triggered by
physical processes that occur more frequently in dense environments, such as
galaxy mergers. Our study demonstrates LAEs as reliable markers of the largest
cosmic structures. When combined with ongoing and upcoming imaging and
spectroscopic surveys, wide-field narrow-band imaging has the potential to
advance our knowledge in the formation and evolution of cosmic structures and
of their galaxy inhabitants.
