Thermoelectrics enabling a direct conversion of waste heat into useful electricity is widely investigated for renewable energy applications. n-Type half-Heusler (HH) MNiSn (M = Ti,Zr,Hf) thermoelectric (TE) elements are known as attractive semiconducting candidates for such purposes. Yet, both electronic and phonon scattering optimization are still required for fulfilling their full potential. In the current research, Ti0.3Zr0.35Hf0.35Ni1.005Sn separating into a main Ti0.3Zr0.35Hf0.35NiSn HH matrix and a minority full-Heusler (FH) Ti0.3Zr0.35Hf0.35Ni2Sn phase is reported. Adverse electronic effects of the metallic FH phase are nearly avoided by its small relative amount and dimension, while maintaining nearly optimal electronic TE performance along with large phonon scattering, minimizing the lattice thermal conductivity. Consequently, a very high maximal TE figure of merit, ZT, of ∼1.04 is obtained, which is among the highest ever reported for n-Type MNiSn HH compounds.