Complex sentence: On his death in 1777 the house was left empty and when his eldest son became the 10th Earl of Kinnoull in 1787 he sold the estate in 1790 to Peter Thellusson (1737--1797).
Split into simpler sentences: On his death in 1777 the house was left empty. His eldest son became the 10th Earl of Kinnoull in 1787 and sold the estate in 1790 to Peter Thellusson (1737--1797).