Simplify the following text for a Grade 8 native English-speaking person.
Original: The following day Domitian was declared emperor by the Praetorian Guard , commencing a reign which lasted fifteen years - longer than any man who had ruled since Tiberius .
Simplified: When Titus died ( he was struck down by a fatal illness in 81 ) the next day Domitian was declared emperor by the Praetorian Guard . This started a reign which lasted fifteen years â `` longer than any man who ruled since Tiberius .