Answer
**A handful of defeats at the hands of the Hephthalite Empire plagued Khosrow I's grandfather, Peroz I.** Peroz I faced relentless military setbacks against the Hephthalites, an empire that exerted dominance over Central Asia during that era. These continuous losses culminated in Peroz I's capture and subsequent execution, illustrating the formidable might of the Hephthalite forces.