  We present optical photometry and spectroscopy of SN\,2019stc
(=ZTF19acbonaa), an unusual Type Ic supernova (SN Ic) at a redshift of
$z=0.117$. SN\,2019stc exhibits a broad double-peaked light curve, with the
first peak having an absolute magnitude of $M_r=-20.0$ mag, and the second
peak, about 80 rest-frame days later, $M_r=-19.2$ mag. The total radiated
energy is large, $E_{\rm rad}\approx 2.5\times 10^{50}$ erg. Despite its large
luminosity, approaching those of Type I superluminous supernovae (SLSNe),
SN\,2019stc exhibits a typical SN Ic spectrum, bridging the gap between SLSNe
and SNe Ic. The spectra indicate the presence of Fe-peak elements, but modeling
of the first light curve peak with radioactive heating alone leads to an
unusually high nickel mass fraction of $f_{\rm Ni}\approx 31\%$ ($M_{\rm
Ni}\approx 3.2$ M$_\odot$). Instead, if we model the first peak with a combined
magnetar spin-down and radioactive heating model we find a better match with
$M_{\rm ej}\approx 4$ M$_\odot$, a magnetar spin period of $P_{\rm spin}\approx
7.2$ ms and magnetic field of $B\approx 10^{14}$ G, and $f_{\rm Ni}\lesssim
0.2$ (consistent with SNe Ic). The prominent second peak cannot be naturally
accommodated with radioactive heating or magnetar spin-down, but instead can be
explained as circumstellar interaction with $\approx 0.7$ $M_\odot$ of
hydrogen-free material located $\approx 400$ AU from the progenitor. Including
the remnant mass leads to a CO core mass prior to explosion of $\approx 6.5$
M$_\odot$. The host galaxy has a metallicity of $\approx 0.26$ Z$_\odot$, low
for SNe Ic but consistent with SLSNe. Overall, we find that SN\,2019stc is a
transition object between normal SNe Ic and SLSNe.
