  We present observations from X-ray to mid-infrared wavelengths of the most
energetic non-quasar transient ever observed, AT2021lwx. Our data show a single
optical brightening by a factor $>100$ to a luminosity of $7\times10^{45}$ erg
s$^{-1}$, and a total radiated energy of $1.5\times10^{53}$ erg, both greater
than any known optical transient. The decline is smooth and exponential and the
ultra-violet - optical spectral energy distribution resembles a black body with
temperature $1.2\times10^4$ K. Tentative X-ray detections indicate a secondary
mode of emission, while a delayed mid-infrared flare points to the presence of
dust surrounding the transient. The spectra are similar to recently discovered
optical flares in known active galactic nuclei but lack some characteristic
features. The lack of emission for the previous seven years is inconsistent
with the short-term, stochastic variability observed in quasars, while the
extreme luminosity and long timescale of the transient disfavour the disruption
of a single solar-mass star. The luminosity could be generated by the
disruption of a much more massive star, but the likelihood of such an event
occurring is small. A plausible scenario is the accretion of a giant molecular
cloud by a dormant black hole of $10^8 - 10^9$ solar masses. AT2021lwx thus
represents an extreme extension of the known scenarios of black hole accretion.
