  We present the discovery with Keck of the extremely infrared (IR) luminous
transient AT 2017gbl, coincident with the Northern nucleus of the luminous
infrared galaxy (LIRG) IRAS 23436+5257. Our extensive multi-wavelength
follow-up spans ~900 days, including photometry and spectroscopy in the optical
and IR, and (very long baseline interferometry) radio and X-ray observations.
Radiative transfer modelling of the host galaxy spectral energy distribution
and long-term pre-outburst variability in the mid-IR indicate the presence of a
hitherto undetected dust obscured active galactic nucleus (AGN). The optical
and near-IR spectra show broad 2000 km/s hydrogen, He I and O I emission
features that decrease in flux over time. Radio imaging shows a fast evolving
compact source of synchrotron emission spatially coincident with AT 2017gbl. We
infer a lower limit for the radiated energy of 7.3 x 10^50 erg from the IR
photometry. An extremely energetic supernova would satisfy this budget, but is
ruled out by the radio counterpart evolution. Instead, we propose AT 2017gbl is
related to an accretion event by the central supermassive black hole, where the
spectral signatures originate in the AGN broad line region and the IR
photometry is consistent with re-radiation by polar dust. Given the fast
evolution of AT 2017gbl, we deem a tidal disruption event (TDE) of a star a
more plausible scenario than a dramatic change in the AGN accretion rate. This
makes AT 2017gbl the third TDE candidate to be hosted by a LIRG, in contrast to
the so far considered TDE population discovered at optical wavelengths and
hosted preferably by post-starburst galaxies.
