  We present a long (150 ks elapsed time) ASCA X-ray observation of the dM3.5e
star EV Lac, during which an exceptionally intense flaring event (lasting
approximately 12 ks) was observed: at the flare's peak, the X-ray count rate in
the ASCA GIS detectors was approx. 300 times the quiescent value. The physical
parameters of the flaring region have been derived by analyzing the decay,
using both a quasi-static approach and an approach based on hydrodynamic
simulations of decaying flaring loops. This second method shows that the
flare's decay is compatible with its being produced in a relatively compact
region of semi-length L approx. 0.5 stellar radii), large but not exceptional
even by solar standards. The flare decays is fast (with a e-folding time for
the light curve of <= 2 ks), but nevertheless the hydrodynamic-based analysis
shows strong evidence for sustained heating, with the shape of the light curve
dominated by the time evolution of the heating rather than by the natural
cooling of the flaring plasma. The quasi-static method yields a much larger
estimate of the loop's length (L approx. 2 stellar radii). The event shows a
light curve characterized by two separate decay time constants (with the
initial decay being faster) and a significant enhancement in the plasma metal
abundance at the peak of the flare. The energetics of the event are
exceptional, with the peak X-ray luminosity of the event reaching up to approx.
25% of the bolometric luminosity of the star, making this one of the largest
X-ray flare (in relative terms) observed to date on a main-sequence star.
