Bound-systems of $Xi^-$--$^{14}_{}{rm N}$ are studied via $Xi^-$ capture at rest followed by emission of a twin single-$Lambda$ hypernucleus in the emulsion detectors. Two events forming extremely deep $Xi^-$ bound states were obtained by analysis of a hybrid method in the E07 experiment at J-PARC and reanalysis of the E373 experiment at KEK-PS. The decay mode of one event was assigned as $Xi^-+^{14}_{}{rm N}to^{5}_{Lambda}{rm He}$+$^{5}_{Lambda}{rm He}$+$^{4}_{}{rm He}$+n. Since there are no excited states for daughter particles, the binding energy of the $Xi^-$ hyperon, $B_{Xi^-}$, in $^{14}_{}{rm N}$ nucleus was uniquely determined to be 6.27 $pm$ 0.27 MeV. Another $Xi^-$--$^{14}_{}{rm N}$ system via the decay $^{9}_{Lambda}{rm Be}$ + $^{5}_{Lambda}{rm He}$ + n brings a $B_{Xi^-}$ value, 8.00 $pm$ 0.77 MeV or 4.96 $pm$ 0.77 MeV, where the two possible values of $B_{Xi^-}$ correspond to the ground and the excited states of the daughter $^{9}_{Lambda}{rm Be}$ nucleus, respectively. Because the $B_{Xi^-}$ values are larger than those of the previously reported events (KISO and IBUKI), which are both interpreted as the nuclear $1p$ state of the $Xi^-$--$^{14}_{}{rm N}$ system, these new events give the first indication of the nuclear $1s$ state of the $Xi$ hypernucleus, $^{15}_{Xi}{rm C}$.
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