Simplify the following text for a Grade 10 native English-speaking person.
Original: Gaius Cassius Longinus ( before 85 BC â `` October 42 BC ) was a Roman senator , a leading instigator of the plot to kill Julius Caesar , and the brother in-law of Marcus Junius Brutus .
Simplified: Gaius Cassius Longinus ( before 85 BC â `` October 42 BC ) was a Roman Senator , a leader of the plot to kill Julius Caesar , and the brother in-law of Brutus .