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We report the discovery of a luminous quasar, J1007+2115 at $z=7.515$("P\={o}niu\={a}'ena"), from our wide-field reionization-era quasar survey.J1007+2115 is the second quasar now known at $z>7.5$, deep into thereionization epoch. The quasar is powered by a $(1.5\pm0.2)\times10^9$$M_{\odot}$ supermassive black hole (SMBH), based on its broad MgIIemission-line profile from Gemini and Keck near-IR spectroscopy. The SMBH inJ1007+2115 is twice as massive as that in quasar J1342+0928 at $z=7.54$, thecurrent quasar redshift record holder. The existence of such a massive SMBHjust 700 million years after the Big Bang significantly challenges models ofthe earliest SMBH growth. Model assumptions of Eddington-limited accretion anda radiative efficiency of 0.1 require a seed black hole of $\gtrsim 10^{4}$$M_{\odot}$ at $z=30$. This requirement suggests either a massive black holeseed as a result of direct collapse or earlier periods of rapid black holegrowth with hyper-Eddington accretion and/or a low radiative efficiency. Wemeasure the damping wing signature imprinted by neutral hydrogen absorption inthe intergalactic medium (IGM) on J1007+2115's Ly$\alpha$ line profile, andfind that it is weaker than that of J1342+0928 and two other $z\gtrsim7$quasars. We estimate an IGM volume-averaged neutral fraction $\langlex\rm_{HI}\rangle=0.39^{+0.22}_{-0.13}$. This range of values suggests a patchyreionization history toward different IGM sightlines. We detect the 158 $\mu$m[C II] emission line in J1007+2115 with ALMA; this line centroid yields asystemic redshift of $z=7.5149\pm0.0004$ and indicates a star formation rate of$\sim210$ $M_{\odot}$ yr$^{-1}$ in its host galaxy.

P\={o}niu\={a}'ena: A Luminous $z=7.5$ Quasar Hosting a 1.5 Billion Solar Mass Black Hole