We have discovered a group of luminous optical transients coincident with the nuclei of their host galaxies. They display a slow and smooth evolution in brightness over a period of several years. Based on extensive monitoring, we are able to exclude active galactic nuclei (AGN) flares and tidal disruption events as the origin of these transients. Rather, we find them to be consistent with superluminous supernovae powered by interaction of the ejecta and circumstellar medium. With the estimated total radiated energy of ~2 x 10^52 erg, our most luminous candidate is potentially the most energetic supernova observed to date. We suggest that the spatial location is an important part of the precursor evolution of these events. It is also likely that transient surveys have ignored these events by associating them contextually with AGN activity.