  We report the discovery and classification of SDSS~J053341.43+001434.1
(SDSS0533), an early-L dwarf first discovered during a powerful $\Delta V <
-11$ magnitude flare observed as part of the ASAS-SN survey. Optical and
infrared spectroscopy indicate a spectral type of L0 with strong H$\alpha$
emission and a blue NIR spectral slope. Combining the photometric distance,
proper motion, and radial velocity of SDSS0533 yields three-dimensional
velocities of $(U,V,W)=(14\pm13,-35\pm14,-94\pm22)$~km~s$^{-1}$, indicating
that it is most likely part of the thick disk population and probably old. The
three detections of SDSS0533 obtained during the flare are consistent with a
total $V$-band flare energy of at least $4.9\times10^{33}$~ergs (corresponding
to a total thermal energy of at least $E_{\rm tot}>3.7\times10^{34}$~erg),
placing it among the strongest detected M dwarf flares. The presence of this
powerful flare on an old L0 dwarf may indicate that stellar-type magnetic
activity persists down to the end of the main sequence and on older ML
transition dwarfs.
