  As part of the Mega MUSCLES Hubble Space Telescope (HST) Treasury program, we
obtained time-series ultraviolet spectroscopy of the M2.5V star, GJ~674. During
the FUV monitoring observations, the target exhibited several small flares and
one large flare (E_FUV = 10^{30.75} ergs) that persisted over the entirety of a
HST orbit and had an equivalent duration >30,000 sec, comparable to the highest
relative amplitude event previously recorded in the FUV. The flare spectrum
exhibited enhanced line emission from chromospheric, transition region, and
coronal transitions and a blue FUV continuum with an unprecedented color
temperature of T_c ~ 40,000+/-10,000 K. In this paper, we compare the flare FUV
continuum emission with parameterizations of radiative hydrodynamic model
atmospheres of M star flares. We find that the observed flare continuum can be
reproduced using flare models but only with the ad hoc addition of hot, dense
emitting component. This observation demonstrates that flares with hot FUV
continuum temperatures and significant EUV/FUV energy deposition will continue
to be of importance to exoplanet atmospheric chemistry and heating even as the
host M dwarfs age beyond their most active evolutionary phases.
