  We report multi-wavelength observations and characterization of the
ultraluminous transient AT 2021lwx (ZTF20abrbeie; aka ``Barbie'') identified in
the alert stream of the Zwicky Transient Facility (ZTF) using a Recommender
Engine For Intelligent Transient Tracking (REFITT) filter on the ANTARES alert
broker. From a spectroscopically measured redshift of 0.995, we estimate a peak
observed pseudo-bolometric luminosity of log (L$_{\text{max}} /
[\text{erg}/\text{s}]$) = 45.7 from slowly fading ztf-$\it{g}$ and ztf-$r$
light curves spanning over 1000 observer-frame days. The host galaxy is not
detected in archival Pan-STARRS observations ($g > 23.3$ mag), implying a lower
limit to the outburst amplitude of more than 5 mag relative to the quiescent
host galaxy. Optical spectra from Lick and Keck Observatories exhibit strong
emission lines with narrow cores from the H Balmer series and ultraviolet
semi-forbidden lines of Si III] $\lambda$1892, C III] $\lambda$1909, and C II]
$\lambda$2325. Typical nebular lines in AGN spectra from ions such as [O II]
and [O III] are not detected. These spectral features, along with the smooth
light curve that is unlike most AGN flaring activity, and the luminosity that
exceeds any observed or theorized supernova, lead us to conclude that AT
2021lwx is most likely an extreme tidal disruption event (TDE). Modeling of ZTF
photometry with MOSFiT suggests that the TDE was between a $\approx 14
M_{\odot}$ star and a supermassive black hole of mass $M_{\text{BH}} \sim$
$10^{8} M_{\odot}$. Continued monitoring of the still-evolving light curve
along with deep imaging of the field once AT 2021lwx has faded can test this
hypothesis and potentially detect the host galaxy.
