We present observations of the extremely luminous but ambiguous nuclear transient (ANT) ASASSN-17jz, spanning roughly 1200 days of the objects 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. The transient peaked at an absolute $B$-band magnitude of $M_{B,{rm peak}}=-22.81$, corresponding to a bolometric luminosity of $L_{rm bol,peak}=8.3times10^{44}$ ergs s$^{-1}$, and exhibited late-time ultraviolet emission with a total emitted energy of $E_{rm tot}=(1.36pm0.08)times10^{52}$ ergs. This late-time light 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, except for the Balmer lines, which became fainter and broader over time. We consider various physical scenarios for the origin of the transient, including those involving supernovae (SNe), tidal disruption event (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.
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