  We present observations of the extremely luminous but ambiguous nuclear
transient (ANT) ASASSN-17jz, spanning roughly 1200 days of the object's
evolution. ASASSN-17jz was discovered by the All-Sky Automated Survey for
Supernovae (ASAS-SN) in the galaxy SDSS J171955.84+414049.4 on UT 2017 July 27
at a redshift of $z=0.1641$. The transient peaked at an absolute $B$-band
magnitude of $M_{B,{\rm peak}}=-22.81$, corresponding to a bolometric
luminosity of $L_{\rm bol,peak}=8.3\times10^{44}$~erg~s$^{-1}$, and exhibited
late-time ultraviolet emission that was still ongoing in our latest
observations. Integrating the full light curve gives a total emitted energy of
$E_{\rm tot}=(1.36\pm0.08)\times10^{52}$~erg, with
$(0.80\pm0.02)\times10^{52}$~erg of this emitted within 200 days of peak light.
This late-time ultraviolet emission is accompanied by increasing X-ray emission
that becomes softer as it brightens. ASASSN-17jz exhibited a large number of
spectral emission lines most commonly seen in active galactic nuclei (AGNs)
with little evidence of evolution. It also showed transient Balmer features
which became fainter and broader over time, and are still being detected
$>1000$ days after peak brightness. We consider various physical scenarios for
the origin of the transient, including supernovae (SNe), tidal disruption
events (TDEs), AGN outbursts, and ANTs. We find that the most likely
explanation is that ASASSN-17jz was an SN~IIn occurring in or near the disk of
an existing AGN, and that the late-time emission is caused by the AGN
transitioning to a more active state.
