Mesoscopic polarization structures such as polar nanoregions and polarization domain walls are the key factors connecting microscopic fundamental polarization structures with macroscopic practical dielectric properties. A snapshot observation of the domain in the relaxor ferroelectrics $\mathrm{Pb}[{(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3})}_{1\text{\ensuremath{-}}x}\mathrm{T}{\mathrm{i}}_{x}]{\mathrm{O}}_{3}$ just in the vicinity of morphotropic phase boundary region was performed by use of 7-ps single shot soft x-ray laser pulse. A self-assembled evolution of the oblique polarization domain was observed in $\mathrm{Pb}[{(\mathrm{M}{\mathrm{g}}_{1/3}\mathrm{N}{\mathrm{b}}_{2/3})}_{72.2}\mathrm{T}{\mathrm{i}}_{27.8}]{\mathrm{O}}_{3}$ under the sample temperature decreased with the thermal equilibrium condition. Based on energetic discussion, antiphase shift of domain wall pairs keeping flat boundaries was proposed for the dielectric response. A sharp enhancement in the dielectric response at the vicinity of morphotropic phase boundary region reported previously was recognized as evidence for the hierarchic nature of the present oblique polarization domain wall.