  We present the detection of an energetic flare on the pre-main sequence M3
star NGTS J121939.5-355557, which we estimate as only 2 Myr old. The flare had
an energy of $3.2\pm^{0.4}_{0.3}\times 10^{36}$erg and a fractional amplitude
of $7.2\pm0.8$, making it one of the most energetic flares seen on an M star.
The star is also X-ray active, in the saturated regime with $log L_{X}/L_{Bol}
= -3.1$. In the flare peak we have identified multi-mode quasi-periodic
pulsations formed of two statistically significant periods of approximately 320
and 660 seconds. This flare is one of the largest amplitude events to exhibit
such pulsations. The shorter period mode is observed to start after a
short-lived spike in flux lasting around 30 seconds, which would not have been
resolved in Kepler or TESS short cadence modes. Our data shows how the high
cadence of NGTS can be used to apply solar techniques to stellar flares and
identify potential causes of the observed oscillations. We also discuss the
implications of this flare for the habitability of planets around M star hosts
and how NGTS can aid in our understanding of this.
