Ammianus Marcellinus Res Gestae val1.5.29

[29] Licinius was sent to Thessalonica; but Constantine, influenced by the example of his father-in-law Herculius Maximianus, ¹ for fear that Licinius might again, with disastrous consequences to the State, resume the purple which he had laid down, and also because the soldiers mutinously demanded his death, had him assassinated at Thessalonica, ² and Martinianus in Cappadocia. Licinius reigned nineteen years and was survived by his wife and a son. And yet, after all the other participants in the abominable persecution ³ had already perished, the penalty he deserved would surely demand this man also, a persecutor so far as he could act as such. ⁴