We report that the RS CVn-type star GT Mus (HR 4492, HD 101379 + HD 101380)
was the most active star in the X-ray sky in the last decade in terms of the
scale of recurrent energetic flares. We detected 11 flares from GT Mus in 8 yr
of observations with Monitor of All-sky X-ray Image (MAXI) from 2009 August to
2017 August. The detected flare peak luminosities were 1-4 $\times$ 10$^{33}$
erg s$^{-1}$ in the 2.0-20.0 keV band for its distance of 109.6 pc. Our timing
analysis showed long durations ($\tau_{\rm r} + \tau_{\rm d}$) of 2-6 days with
long decay times ($\tau_{\rm d}$) of 1-4 days. The released energies during the
decay phases of the flares in the 0.1-100 keV band ranged 1-11 $\times$
10$^{38}$ erg, which are at the upper end of the observed stellar flare. The
released energies during whole duration time ranged 2-13 $\times$ 10$^{38}$ erg
in the same band. We carried out X-ray follow-up observations for one of the 11
flares with Neutron star Interior Composition Explorer (NICER) on 2017 July 18
and found that the flare cooled quasi-statically. On the basis of a
quasi-static cooling model, the flare loop length is derived to be 4 $\times$
10$^{12}$ cm (or 60 R$_{\odot}$). The electron density is derived to be 1
$\times$ 10$^{10}$ cm$^{-3}$, which is consistent with the typical value of
solar and stellar flares (10$^{10-13}$ cm$^{-3}$). The ratio of the cooling
timescales between radiative cooling ($\tau_{\rm rad}$) and conductive cooling
($\tau_{\rm cond}$) is estimated to be $\tau_{\rm rad}$ $\sim$ 0.1$\tau_{\rm
cond}$ from the temperature; thus radiative cooling was dominant in this flare.