Simplify the following text for a Grade 12 native English-speaking person.
Original: The territory of the Duchy of Westphalia , which Hesse-Darmstadt received from the 1803 Reichsdeputationshauptschluss , was given to the Kingdom of Prussia .
Simplified: The territory of the Duchy of Westphalia , which Hesse-Darmstadt received from the 1803 German Mediatisation , was ceded to the Kingdom of Prussia .