  SDSS J015957.64+003310.5 is an X-ray selected, $z=0.31$ AGN from the Stripe
82X survey that transitioned from a Type 1 quasar to a Type 1.9 AGN between
2000 and 2010. This is the most distant AGN, and first quasar, yet observed to
have undergone such a dramatic change. We re-observed the source with the
double spectrograph on the Palomar 5m telescope in July 2014 and found that the
spectrum is unchanged since 2010. From fitting the optical spectra, we find
that the AGN flux dropped by a factor of 6 between 2000 and 2010 while the
broad H$\alpha$ emission faded and broadened. Serendipitous X-ray observations
caught the source in both the bright and dim state, showing a similar 2-10 keV
flux diminution as the optical while lacking signatures of obscuration. The
optical and X-ray changes coincide with $g$-band magnitude variations over
multiple epochs of Stripe 82 observations. We demonstrate that variable
absorption, as might be expected from the simplest AGN unification paradigm,
does not explain the observed photometric or spectral properties. We interpret
the changing state of J0159+0033 to be caused by dimming of the AGN continuum,
reducing the supply of ionizing photons available to excite gas in the
immediate vicinity around the black hole. J0159+0033 provides insight into the
intermittency of black hole growth in quasars, as well as an unprecedented
opportunity to study quasar physics (in the bright state) and the host galaxy
(in the dim state), which has been impossible to do in a single sources until
now.
