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.
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