Abstract: One of the most important mysteries in the field of galaxy evolution is when, where, and how the most massive galaxies formed and assembled their mass in the early Universe. Early ultra-massive galaxies (UMGs) place strong constraints on models for galaxy formation and evolution. Large near-infrared photometric surveys have revealed numerous UMG candidates at increasingly high redshifts through the past decade, but spectroscopic followup is necessary to confirm their nature. In this talk I will first provide an overview of the MAGAZ3NE spectroscopic survey, which targeted UMG candidates to characterize their stellar populations and their environments. I will then discuss the three protoclusters we discovered around UMGs and the insights they give to UMG evolution. Finally, I will present results from MAGAZ3NE’s spectroscopic campaign searching for the reddest, most massive UMG candidates and the subsequent implications for the extremely high-mass end of the stellar mass function.