Simplify the following text for a Grade 6 native English-speaking person.
Original: Arminius , also known as Armin or Hermann ( b. 18 BC\/17 BC in Magna Germania ; d. AD 21 in Germania ) was a chieftain of the Cherusci who defeated a Roman army in the Battle of the Teutoburg Forest .
Simplified: Arminius , also known as Armin or Hermann ( b. 18 BC\/17 BC in Magna Germania ; d. AD 21 in Germania ) was a chieftain of the Cherusci .