Quasars are actively accreting black holes, with masses of more than a billion suns, at the hearts of distant galaxies. Because quasars are the most luminous long-lived phenomena in cosmic history, they can be used to observe gigantic volumes of the Universe at redshifts back to almost the dawn of time. As part of the fourth iteration of the Sloan Digital Sky Survey (SDSS-IV) the extended Baryon Oscillation Spectroscopic Survey (eBOSS) will identify over half-a-million new quasars over a wide range of redshifts close to the faint limits of the SDSS imaging. I will characterize the eBOSS quasar sample based on data from the Sloan Extended QUasar, ELG and LRG Survey (SEQUELS), a ~300 square degree precursor survey near the North Galactic Cap. I will outline my group's recent work on using CMB lensing maps to directly "weigh" the environments around quasars, and will discuss the prospects of eBOSS for characterizing the masses of the dark matter halos that host quasars. Over the next 5 years, eBOSS will use the most voluminous maps ever made, etched in quasar-light, to probe dark matter, dark energy and the evolution of galaxies throughout cosmic history.