Enhanced piezoelectric, dielectric properties and thermal stability in ternary relaxor-PbTiO3 based ferroelectric crystals are expected to develop the next-generation of electromechanical devices. However, due to their increased disorder compared to other ferroelectrics, designing a controllable phase boundary structure and engineered domain remains a challenging task. Here, we construct a monoclinic heterophase coexisting in a ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal with optimized composition and an ultrahigh piezoelectric coefficient of 1400 pC N-1, to quantify the correlation between spontaneous nanopolarity and phase heterogeneity, in an attempt to understand the origin of the exceptional functionalities. By designing an in situ high-resolution spectroscopic-microscopic technique, we have observed Ma and Mc heterophase mixtures spatially separated by the monoclinic heterophase boundary (MHB), which are responsible for the ferroelectric-dominated and relaxor-ferroelectric-dominated nanodomain structure, respectively. Internal energy mapping from optical soft mode dynamics reveals the inhomogeneous polarization and local symmetry on both sides of the MHB. Various molecular polarizabilities and localized octahedral distortions correlate directly with monoclinic regions and electromechanical contribution. This work clarifies the heterogeneity between structure, energy, and polar order and provides a new design freedom for advanced relaxor ferroelectrics.