  We present the discovery of a gravitationally lensed dust-reddened QSO at z =
2.517, identified in a survey for QSOs by infrared selection. Hubble Space
Telescope imaging reveals a quadruply lensed system in a cusp configuration,
with a maximum image separation of ~1.8\arcsec. We find that compared to the
central image of the cusp, the neighboring brightest image is anomalous by a
factor of ~ 7 - 10, which is the largest flux anomaly measured to date in a
lensed QSO. Incorporating high-resolution Jansky Very Large Array radio imaging
and sub-mm imaging with the Atacama Large (sub-)Millimetre Array, we conclude
that a low-mass perturber is the most likely explanation for the anomaly. The
optical through near-infrared spectrum reveals that the QSO is moderately
reddened with E(B - V) = 0.7 - 0.9. We see an upturn in the ultraviolet
spectrum due to ~ 1% of the intrinsic emission being leaked back into the line
of sight, which suggests that the reddening is intrinsic and not due to the
lens. The QSO may have an Eddington ratio as high as L/L_Edd ~ 0.2. Consistent
with previous red QSO samples, this source exhibits outflows in its spectrum as
well as morphological properties suggestive of it being in a merger-driven
transitional phase. We find a host-galaxy stellar mass of log M_*/M_Sun = 11.4,
which is higher than the local M_BH vs. M_* relation, but consistent with other
high redshift QSOs. When de-magnified, this QSO is at the knee of the
luminosity function, allowing for the detailed study of a more typical
moderate-luminosity infrared-selected QSO at high redshift.
