  We present optical and near-infrared observations of SN 2012au, a
slow-evolving supernova (SN) with properties that suggest a link between
subsets of energetic and H-poor SNe and superluminous SNe. SN 2012au exhibited
conspicuous SN Ib-like He I lines and other absorption features at velocities
reaching 2 x 10^4 km/s in its early spectra, and a broad light curve that
peaked at M_B = -18.1 mag. Models of these data indicate a large explosion
kinetic energy of 10^{52} erg and 56Ni mass ejection of 0.3 Msolar on par with
SN 1998bw. SN 2012au's spectra almost one year after explosion show a blend of
persistent Fe II P-Cyg absorptions and nebular emissions originating from two
distinct velocity regions. These late-time emissions include strong [Fe II],
[Ca II], [O I], Mg I], and Na I lines at velocities > 4500 km/s, as well as O I
and Mg I lines at noticeably smaller velocities of 2000 km/s. Many of the
late-time properties of SN 2012au are similar to the slow-evolving hypernovae
SN 1997dq and SN 1997ef, and the superluminous SN 2007bi. Our observations
suggest that a single explosion mechanism may unify all of these events that
span -21 < M_B < -17 mag. The aspherical and possibly jetted explosion was most
likely initiated by the core collapse of a massive progenitor star and created
substantial high-density, low-velocity Ni-rich material.
