  White-light superflares from ultra cool stars are thought to be resulted from
magnetic reconnection, but the magnetic dynamics in a fully convective star is
not clear yet. In this paper, we report a stellar superflare detected with the
Ground Wide Angle Camera (GWAC), along with rapid follow-ups with the F60A,
Xinglong 2.16m and LCOGT telescopes. The effective temperature of the
counterpart is estimated to be $2200\pm50$K by the BT-Settl model,
corresponding to a spectral type of L0. The $R-$band light curve can be modeled
as a sum of three exponential decay components, where the impulsive component
contributes a fraction of 23\% of the total energy, while the gradual and the
shallower decay phases emit 42\% and 35\% of the total energy, respectively.
The strong and variable Balmer narrow emission lines indicate the large
amplitude flare is resulted from magnetic activity. The bolometric energy
released is about $6.4\times10^{33}$ ergs, equivalent to an energy release in a
duration of 143.7 hours at its quiescent level. The amplitude of $\Delta R=-8.6
$mag ( or $\Delta V=-11.2$ mag), placing it one of the highest amplitudes of
any ultra cool star recorded with excellent temporal resolution. We argue that
a stellar flare with such rapidly decaying and huge amplitude at distances
greater than 1 kpc may be false positive in searching for counterparts of
catastrophic events such as gravitational wave events or gamma-ray bursts,
which are valuable in time-domain astronomy and should be given more attention.

Solar and Stellar Astrophysics;High Energy Astrophysical Phenomena