We present the discovery of ASASSN-18jd (AT 2018bcb), a luminous optical/UV/X-ray transient located in the nucleus of the galaxy 2MASX J22434289$-$1659083 at $z=0.1192$. Swift UVOT photometry shows the UV SED of the transient to be well modeled by a slowly shrinking blackbody with temperature $T sim 2.5 times 10^{4} rm ~K$, a maximum observed luminosity of $L_text{max} = 4.5^{+0.6}_{-0.3} times 10^{44} rm ~erg ~s^{-1}$, and a total radiated energy of $E = 9.6^{+1.1}_{-0.6} times 10^{51} rm ~erg$. X-ray data from Swift XRT and XMM-Newton show a transient, variable X-ray flux with blackbody and power-law components. Optical spectra show strong, roughly constant broad Balmer emission as well as transient features attributable to He II, N III-V, O III, and coronal Fe. While ASASSN-18jd shares similarities with Tidal Disruption Events (TDEs), it is also similar to the rapid turn-on events seen in quiescent galaxies and in faint Active Galactic Nuclei (AGNs).
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