  We present optical and near-infrared photometry and spectroscopy of SN
2023aew and our findings on its remarkable properties. This event, initially
resembling a Type IIb supernova (SN), rebrightens dramatically $\sim$90 d after
the first peak, at which time its spectrum transforms into that of a SN Ic. The
slowly evolving spectrum specifically resembles a post-peak SN~Ic with
relatively low line velocities even during the second rise. The second peak,
reached 119 d after the first peak, is both more luminous ($M_r =
-18.75\pm0.04$ mag) and much broader than those of typical SNe Ic. Blackbody
fits to SN 2023aew indicate that the photosphere shrinks almost throughout its
observed evolution, and the second peak is caused by an increasing temperature.
Bumps in the light curve after the second peak suggest interaction with
circumstellar matter (CSM) or possibly accretion. We consider several scenarios
for producing the unprecedented behavior of SN 2023aew. Two separate SNe,
either unrelated or from the same binary system, require either an incredible
coincidence or extreme fine-tuning. A pre-SN eruption followed by a SN requires
an extremely powerful, SN-like eruption (consistent with $\sim$10$^{51}$ erg)
and is also disfavored. We therefore consider only the first peak a true
stellar explosion. The observed evolution is difficult to reproduce if the
second peak is dominated by interaction with a distant CSM shell. A delayed
internal heating mechanism is more likely, but emerging embedded interaction
with a CSM disk should be accompanied by CSM lines in the spectrum, which are
not observed, and is difficult to hide long enough. A magnetar central engine
requires a delayed onset to explain the long time between the peaks. Delayed
fallback accretion onto a black hole may present the most promising scenario,
but we cannot definitively establish the power source.
