Simplify the following text for a Grade 6 native English-speaking person.
Original: The oldest member of the House of Wettin who is known for certain is Dietrich I von Wettin , also known as Thiedericus , or as Thierry I of Liesgau ( died c. 982 ) .
Simplified: The oldest known member of the House of Wettin was Thiedericus ( died 982 ) .