Simplify the following text for a Grade 5 native English-speaking person.
Original: The elder Pretorius had become a national hero of the Voortrekkers after his victory over the Zulus in the Battle of Blood River . Andries Pretorius also negotiated the Sand River Convention ( 1852 ) , in which Britain acknowledged the independence of the Transvaal .
Simplified: Andries Pretorius had become a hero of the Voortrekkers after the Battle of Blood River .