Simplify the following text for a Grade 10 native English-speaking person.
Original: An equestrian by birth , Sejanus rose to power as prefect of the Roman imperial bodyguard , known as the Praetorian Guard , of which he was commander from AD 14 until his death in 31 .
Simplified: An equestrian by birth , Sejanus rose to power as Praetorian Prefect . He commanded the Roman imperial bodyguard , the Praetorian Guard , from AD 14 until his death in 31 .