  Supernova (SN) 2015bh (or SNhunt275) was discovered in NGC 2770 on 2015
February with an absolute magnitude of Mr ~ -13.4 mag, and was initially
classified as a SN impostor. Here we present the photometric and spectroscopic
evolution of SN 2015bh from discovery to late phases (~ 1 yr after). In
addition, we inspect archival images of the host galaxy up to ~ 21 yr before
discovery, finding a burst ~ 1 yr before discovery, and further signatures of
stellar instability until late 2014. Later on, the luminosity of the transient
slowly increases, and a broad light curve peak is reached after about three
months. We propose that the transient discovered in early 2015 could be a
core-collapse SN explosion. The pre-SN luminosity variability history, the
long-lasting rise and faintness first light curve peak suggests that the
progenitor was a very massive, unstable and blue star, which exploded as a
faint SN because of severe fallback of material. Later on, the object
experiences a sudden brightening of 3 mag, which results from the interaction
of the SN ejecta with circumstellar material formed through repeated past
mass-loss events. Spectroscopic signatures of interaction are however visible
at all epochs. A similar chain of events was previously proposed for the
similar interacting SN 2009ip.
