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We present nebular-phase imaging and spectroscopy for the hydrogen-poorsuperluminous supernova SN 2015bn, at redshift z=0.1136, spanning +250-400 dafter maximum light. The light curve exhibits a steepening in the decline ratefrom 1.4 mag/(100 d) to 1.7 mag/(100 d), suggestive of a significant decreasein the opacity. This change is accompanied by a transition from a bluecontinuum superposed with photospheric absorption lines to a nebular spectrumdominated by emission lines of oxygen, calcium and magnesium. There are noobvious signatures of circumstellar interaction or large nickel mass. We showthat the spectrum at +400 d is virtually identical to a number of energeticType Ic supernovae such as SN 1997dq, SN 2012au, and SN 1998bw, indicatingsimilar core conditions and strengthening the link between `hypernovae'/longgamma-ray bursts and superluminous supernovae. A single explosion mechanism mayunify these events that span absolute magnitudes of -22 < M_B < -17. Both thelight curve and spectrum of SN 2015bn are consistent with an engine-drivenexplosion ejecting 7-30 M$_\odot$ of oxygen-dominated ejecta (for reasonablechoices in temperature and opacity). A strong and relatively narrow O II$\lambda$7774 line, seen in a number of these energetic events but not innormal supernovae, may point to an inner shell that is the signature of acentral engine.

Superluminous supernova 2015bn in the nebular phase: evidence for the engine-powered explosion of a stripped massive star