  We present the first detection of an X-ray flare from an ultracool dwarf of
spectral class L. The event was identified in the EXTraS database of XMM-Newton
variable sources, and its optical counterpart, J0331-27, was found through a
cross-match with the Dark Energy Survey Year 3 release. Next to an earlier
four-photon detection of Kelu-1, J0331-27 is only the second L dwarf detected
in X-rays, and much more distant than other ultracool dwarfs with X-ray
detections (photometric distance of 240 pc). From an optical spectrum with the
VIMOS instrument at the VLT, we determine the spectral type of J0331-27 to be
L1. The X-ray flare has an energy of E_X,F ~ 2x10^33 erg, placing it in the
regime of superflares. No quiescent emission is detected, and from 2.5 Msec of
XMM data we derive an upper limit of L_X,qui < 10^27 erg/s. The flare peak
luminosity L_X,peak = 6.3x10^29 erg/s, flare duration tau_decay ~ 2400 s, and
plasma temperature (~16 MK) are similar to values observed in X-ray flares of M
dwarfs. This shows that strong magnetic reconnection events and the ensuing
plasma heating are still present even in objects with photospheres as cool as
~2100 K. However, the absence of any other flares above the detection threshold
of E_X,F ~2.5x10^32 erg in a total of ~2.5 Ms of X-ray data yields a flare
energy number distribution inconsistent with the canonical power law dN/dE ~
E^-2, suggesting that magnetic energy release in J0331-27 -- and possibly in
all L dwarfs -- takes place predominantly in the form of giant flares.
