  Protoclusters are the densest regions in the distant universe ($z>2$) and are
the progenitors of massive galaxy clusters ($M_{halo}>10^{14}{\rm M}_\odot$) in
the local universe. They undoubtedly play a key role in early massive galaxy
evolution and they may host the earliest sites of galaxy quenching or even
induce extreme states of star formation. Studying protoclusters therefore not
only gives us a window into distant galaxy formation but also provides an
important link in our understanding of how dense structures grow over time and
modify the galaxies within them. Current protocluster samples are completely
unable to address these points because they are small and selected in a
heterogeneous way. We propose the Roman-Cosmic Noon survey, whose centerpiece
is an extremely deep (30ksec) and wide area (10 deg$^2$) prism slitless
spectroscopy survey to identify the full range of galaxy structures at $2<z<3$.
This survey will include 1500 uniformly selected protoclusters, their
surrounding cosmic web environments, and at least 15,000 protocluster galaxies
with $M_\star>10^{10.5} {\rm M}_\odot$ across the full range of star formation
histories as well as many more lower mass star-forming galaxies. The survey
will also contain field galaxies to much lower masses than in the High Latitude
Wide Area Survey, but over an area dwarfing any current or planned deep
spectroscopy probe at $z>2$. With the prism spectroscopy and some modest
additional imaging this survey will measure precise stellar mass functions,
quenched fractions, galaxy and protocluster morphologies, stellar ages,
emission-line based SFRs, and metallicities. It will have extensive legacy
value well beyond the key protocluster science goals.
