Complex sentence: Sempad seized the throne with the aid of his brother Constantine while his brothers Hethum II and Thoros were in Constantinople, and on Hethum's return, Sempad had Hethum blinded by cauterization and both brothers imprisoned at Partzerpert.
Split into simpler sentences: Sempad seized the throne with the aid of his brother Constantine while his brothers Hethum II and Thoros were in Constantinople. On Hethum's return, Sempad had Hethum blinded by cauterization and both brothers imprisoned at Partzerpert.