  We report the discovery of an exceptional MIR flare in a Type 2 AGN, SDSS
J165726.81+234528.1, at $z=0.059$. This object brightened by 3 mag in the
Wide-field Infrared Survey Explorer (WISE) $W1$ and $W2$ bands between 2015 and
2017 (and is fading since 2018), without significant changes ($\lesssim$ 0.2
mag) in the optical over the same period of time. Based on the WISE light
curves and near-IR imaging, the flare is more significant at longer
wavelengths, suggesting an origin of hot dust emission. The estimated black
hole mass ($\sim 10^{6.5}\,M_\odot$) from different methods places its peak
bolometric luminosity around the Eddington limit. The high luminosity of the
MIR flare and its multi-year timescale suggest that it most likely originated
from reprocessed dust radiation in an extended torus surrounding the AGN,
instead of from stellar explosions. The MIR color variability is consistent
with known changing-look AGN and tidal disruption events (TDEs), but
inconsistent with normal supernovae. We suggest that it is a turning-on Type 2
AGN or TDE, where the optical variability is obscured by the dust torus during
the transition. This MIR flare event reveals a population of dramatic nuclear
transients that are missed in the optical.
