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 . 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 .