/ AbstractBound-systems of $Ξ^-$--$^{14}_{}{\rm N}$ are studied via $Ξ^-$ capture at rest followed by emission of a twin single-$Λ$ hypernucleus in the emulsion detectors. Two events forming extremely deep $Ξ^-$ 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 $Ξ^-+^{14}_{}{\rm N}\to^{5}_Λ{\rm He}$+$^{5}_Λ{\rm He}$+$^{4}_{}{\rm He}$+n. Since there are no excited states for daughter particles, the binding energy of the $Ξ^-$ hyperon, $B_{Ξ^-}$, in $^{14}_{}{\rm N}$ nucleus was uniquely determined to be 6.27 $\pm$ 0.27 MeV. Another $Ξ^-$--$^{14}_{}{\rm N}$ system via the decay $^{9}_Λ{\rm Be}$ + $^{5}_Λ{\rm He}$ + n brings a $B_{Ξ^-}$ value, 8.00 $\pm$ 0.77 MeV or 4.96 $\pm$ 0.77 MeV, where the two possible values of $B_{Ξ^-}$ correspond to the ground and the excited states of the daughter $^{9}_Λ{\rm Be}$ nucleus, respectively. Because the $B_{Ξ^-}$ values are larger than those of the previously reported events (KISO and IBUKI), which are both interpreted as the nuclear $1p$ state of the $Ξ^-$--$^{14}_{}{\rm N}$ system, these new events give the first indication of the nuclear $1s$ state of the $Ξ$ hypernucleus, $^{15}_Ξ{\rm C}$.