  We present the discovery of ASASSN-18ey (MAXI J1820+070), a new black hole
low-mass X-ray binary discovered by the All-Sky Automated Survey for SuperNovae
(ASAS-SN). A week after ASAS-SN discovered ASASSN-18ey as an optical transient,
it was detected as an X-ray transient by MAXI/GCS. Here, we analyze ASAS-SN and
Asteroid Terrestrial-impact Last Alert System (ATLAS) pre-outburst optical
light curves, finding evidence of intrinsic variability for several years prior
to the outburst. While there was no long-term rise leading to outburst, as has
been seen in several other systems, the start of the outburst in the optical
preceded that in the X-rays by $7.20\pm0.97~\rm days$. We analyze the
spectroscopic evolution of ASASSN-18ey from pre-maximum to $> 100~\rm days$
post-maximum. The spectra of ASASSN-18ey exhibit broad, asymmetric,
double-peaked H$\alpha$ emission. The Bowen blend ($\lambda \approx 4650$\AA)
in the post-maximum spectra shows highly variable double-peaked profiles,
likely arising from irradiation of the companion by the accretion disk, typical
of low-mass X-ray binaries. The optical and X-ray luminosities of ASASSN-18ey
are consistent with black hole low-mass X-ray binaries, both in outburst and
quiescence.
