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We report the discovery of Swift J221951-484240 (hereafter: J221951), aluminous slow-evolving blue transient that was detected by the Neil GehrelsSwift Observatory Ultra-violet/Optical Telescope (Swift/UVOT) during thefollow-up of Gravitational Wave alert S190930t, to which it is unrelated.Swift/UVOT photometry shows the UV spectral energy distribution of thetransient to be well modelled by a slowly shrinking black body with anapproximately constant temperature of T~2.5x10^4 K. At a redshift z=0.5205,J221951 had a peak absolute magnitude of M_u,AB = -23 mag, peak bolometricluminosity L_max=1.1x10^45 erg s^-1 and a total radiated energy of E>2.6x10^52erg. The archival WISE IR photometry shows a slow rise prior to a peak near thediscovery date. Spectroscopic UV observations display broad absorption lines inN V and O VI, pointing toward an outflow at coronal temperatures. The lack ofemission in the higher H~Lyman lines, N I and other neutral lines is consistentwith a viewing angle close to the plane of the accretion or debris disc. Theorigin of J221951 can not be determined with certainty but has propertiesconsistent with a tidal disruption event and the turn-on of an active galacticnucleus.

Swift/UVOT discovery of Swift J221951-484240: a UV luminous ambiguous nuclear transient