With Agrippina 's influence over her son severed , she reportedly began pushing for Britannicus , Nero 's stepbrother , to become emperor . Nearly fourteen @-@ year @-@ old Britannicus , heir @-@ designate prior to Nero 's adoption , was still legally a minor , but was approaching legal adulthood . According to Tacitus , Agrippina hoped that with her support , Britannicus , being the blood son of Claudius , would be seen as the true heir to the throne by the state over Nero . However , the youth died suddenly and suspiciously on 12 February 55 , the very day before his proclamation as an adult had been set .