  We present observations of supernova (SN) 2017ens, discovered by the ATLAS
survey and identified as a hot blue object through the GREAT program. The
redshift z=0.1086 implies a peak brightness of M_g=-21.1 mag, placing the
object within the regime of superluminous supernovae. We observe a dramatic
spectral evolution, from initially being blue and featureless, to later
developing features similar to those of the broadlined Type Ic SN 1998bw, and
finally showing ~2000 km s^-1 wide H-alpha and H-beta emission. Relatively
narrow Balmer emission (reminiscent of a SN IIn) is present at all times. We
also detect coronal lines, indicative of a dense circumstellar medium. We
constrain the progenitor wind velocity to ~50-60 km s^-1 based on P-Cygni
profiles, which is far slower than those present in Wolf-Rayet stars. This may
suggest that the progenitor passed through a luminous blue variable phase, or
that the wind is instead from a binary companion red supergiant star. At late
times we see the ~2000 km s^-1 wide H-alpha emission persisting at high
luminosity (~3x10^40 erg s^-1) for at least 100 day, perhaps indicative of
additional mass loss at high velocities that could have been ejected by a
pulsational pair instability.
