Simplify the following text for a Grade 13 native English-speaking person.
Original: It was common for the Ernestine duchies to merge and split ; Ernest 's combined duchy was divided again after his death in 1675 , and the Duchy of Saxe-Gotha-Altenburg proper came into existence in 1680 with the completion of this division and the accession of his eldest son , Frederick to the subdivision centered around the towns of Gotha and Altenburg .
Simplified: The duchy was split up again after Ernestâ s death in 1675 , and the Duchy of Saxe-Gotha-Altenburg officially came into existence in 1680 when his eldest son , Frederick , took control of the parts of the country Gotha and Altenburg .