  Recent all-sky surveys have led to the discovery of new types of transients.
These include stars disrupted by the central supermassive black hole, and
supernovae that are 10-100 times more energetic than typical ones. However, the
nature of even more energetic transients that apparently occur in the innermost
regions of their host galaxies is hotly debated. Here we report the discovery
of the most energetic of these to date: PS1-10adi, with a total radiated energy
of ~ 2.3 x 10^52 erg. The slow evolution of its light curve and persistently
narrow spectral lines over ~3 yr are inconsistent with known types of recurring
black hole variability. The observed properties imply powering by shock
interaction between expanding material and large quantities of surrounding
dense matter. Plausible sources of this expanding material are a star that has
been tidally disrupted by the central black hole, or a supernova. Both could
satisfy the energy budget. For the former, we would be forced to invoke a new
and hitherto unseen variant of a tidally disrupted star, while a supernova
origin relies principally on environmental effects resulting from its nuclear
location. Remarkably, we also discovered that PS1-10adi is not an isolated
case. We therefore surmise that this new population of transients has
previously been overlooked due to incorrect association with underlying central
black hole activity.
