A Metal-Strong and Dust-Rich Damped \lya Absorption System toward the
  Quasar SDSS J115705.52+615521.7
  (Abridged for arXiv) We report the discovery of an unusual, extremely
dust-rich and metal-strong damped \lya absorption system (DLA) at a redshift
$z_{a}=2.4596$ toward the quasar SDSS J115705.52+615521.7 (hereafter
J1157+6155) with an emission-line redshift $z_{e}=2.5125$. Its neutral hydrogen
column density $N_{\hi} = 10^{21.8\pm0.2}$ cm$^{-2}$ is among the highest
values measured in quasar DLAs. The measured metal column density is
$N_{ZnII}\approx 10^{13.8}$ cm$^{-2}$, which is about 1.5 times larger than the
largest value in any previously observed quasar DLAs. The best-fit curve is a
MW-like law with a significant broad feature centered around 2175 {\AA} in the
rest frame of the absorber. The measured extinction $A_V \approx 0.92$ mag is
unprecedentedly high in quasar DLAs. After applying an extinction correction,
the $i$ band absolute magnitude of the quasar is as high as $M_{i} \approx
-29.4$ mag, placing it one of the most luminous quasars ever known. This
discovery is suggestive of the existence of a rare yet important population of
dust-rich DLAs with both high metallicities and high column densities, which
may have significant impact on the measurement of the cosmic evolution of
neutral gas mass density and metallicity.
