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The Hyperion proto-supercluster at zvirgul2.5 is the largest and best-studied large-scale structure observed during the peak of the cosmic star formation activity. Hyperion is the most massive overdensity yet discovered in the high-redshift universe comma comparable in size and mass to present-day superclusters comma and is comprised of seven distinct matter peaks comma each corresponding to a proto-group or -cluster exhibiting its own unique activity. These peaks span over an order of magnitude in total mass and show a variety of levels of dynamical maturity. A small portion of Hyperion was serendipitously observed by the 3D-HST survey comma revealing tantalizing indications that stellar mass assembly was proceeding rapidly in member galaxies both through star formation and merging processes. We propose to use HST.s unique capabilities comma combined with existing data comma to perform unbiased studies of environmentally-driven evolution in the early universe. Our program will produce a near complete map of Hyperion with WFC3 spectroscopy and imaging comma confirming virgul150 members (virgul50 in the densest peaks) - doubling the member sample and uniquely sampling the dusty star-forming and quiescent members. With this incredible sample comma we will constrain the relationships between average galaxy star formation rates comma stellar mass functions comma and merging activity and environment. In conjunction with simulations and semi-empirical models comma we will place strong constrains on different avenues of stellar mass assembly at a time when environmental quenching should be imposing itself in earnest and the vast majority of the stellar content of galaxies should be forming.