Reducing the lattice thermal conductivity (κ_L) comprises one of the crucial aspects of thermoelectric research. Ternary intermetallic half Heusler compounds have revealed properties promising for thermoelectric applications. Studies have shown that self doping with Ni in Ni based half Heuslers leads to unprecedented lowering in the κ_L. Although the underlying physical mechanisms have not been explored in detail, with ZrNiSn as a case study, we experimentally investigate the change in κ_L with increase in the Ni concentration in Ni based n-type half Heusler alloys. We observe that at excess Ni doping of 3% in the half Heusler lattice, the thermal conductivity reduces by more than 60%. Our density functional theory based analysis on the ongoing phenomena reveals that at ultralow Ni doping, the localized modes of the antisite Ni defect hybridize with the acoustic modes and this plays the most dominant role in scattering of the thermal phonons leading to significant lowering in κL. Our theoretical analysis can be employed for predicting a suitable dopants that may reduce the κL prior to the synthesis of the compound in the laboratory.