  We present observations of a peculiar hydrogen- and helium-poor
stripped-envelope (SE) supernova (SN) 2020wnt, primarily in the optical and
near-infrared (near-IR). Its peak absolute bolometric magnitude of -20.9 mag
and a rise time of 69~days are reminiscent of hydrogen-poor superluminous SNe
(SLSNe~I), luminous transients potentially powered by spinning-down magnetars.
Before the main peak, there is a brief peak lasting <10 days post-explosion,
likely caused by interaction with circumstellar medium (CSM) ejected ~years
before the SN explosion. The optical spectra near peak lack a hot continuum and
OII absorptions, which are signs of heating from a central engine; they
quantitatively resemble those of radioactivity-powered H/He-poor Type Ic SESNe.
At ~1 year after peak, nebular spectra reveal a blue pseudo-continuum and
narrow OI recombination lines associated with magnetar heating. Radio
observations rule out strong CSM interactions as the dominant energy source at
+266 days post peak. Near-IR observations at +200-300 day reveal carbon
monoxide and dust formation, which causes a dramatic optical light curve dip.
Pair-instability explosion models predict slow light curve and spectral
features incompatible with observations. SN 2020wnt is best explained as a
magnetar-powered core-collapse explosion of a 28 Msun pre-SN star. The
explosion kinetic energy is significantly larger than the magnetar energy at
peak, effectively concealing the magnetar-heated inner ejecta until well after
peak. SN 2020wnt falls into a continuum between normal SNe Ic and SLSNe I and
demonstrates that optical spectra at peak alone cannot rule out the presence of
a central engine.
