All the youth believed Antoninus’ promises and credited the probability of what he said because he had already honoured the city. They arrived with their parents and brothers, who were equally pleased at the prospects for their relations. Antoninus inspected the ranks as they stood there and passed from man to man, saying a different word of encouragement to each man as he came up to him, while the entire army encircled them without being noticed or rousing suspicion. After he had gone up and down all the ranks, he judged they were by this time surrounded by arms like animals trapped in a net. So he actually left the field with his personal bodyguard, while from every side the soldiers at a single signal fell upon all the encircled young men and any who were there for other reasons. They wiped them out with every kind of slaughter, armed soldiers against defenceless men who were totally surrounded.
Edition :
Herodian. History of the Empire, Volume I: Books 1-4. Translated by C. R. Whittaker. Loeb Classical Library 454. Cambridge, MA: Harvard University Press, 1969.
Herodian. History of the Empire, Volume II: Books 5-8. Translated by C. R. Whittaker. Loeb Classical Library 455. Cambridge, MA: Harvard University Press, 1970.
Remark :
motive: Caracalla wanted to punish the Alexandrians for reports of disrespectful humour. (4.9.1-5)