Emperor Constantine had a son Flavius Julius Crispus (b. 305), who was a co-regent with his father until his unfortunate death in 326.
Perhaps the best and most accurate summary of Crispus' death is given at Roman Emperors.org, (from Hans Pohlsander) as far as it can be substantiated and documented:
"Crispus' end was as tragic as his career had been brilliant. His own father ordered him to be put to death. (16) We know the year of this sad event, 326, from the Consularia Constantinopolitana,(17) and the place, Pola in Istria, from Ammianus Marcellinus. (18) The circumstances, however, are less clear. Zosimus (6th c.) (19) and Zonaras (12th c.) (20) both report that Crispus and his stepmother Fausta were involved in an illicit relationship. There may be as much gossip as fact in their reports, but it is certain that at some time during the same year the emperor ordered the death of his own wife as well, (21) and the two cases must be considered together. (22) That Crispus and Fausta plotted treason is reported by Gregory of Tours, (23) but not very believable. We must resolutely reject the claim of Zosimus (24) that it was Constantine's sense of guilt over these deeds which caused him to accept Christianity, as it alone promised him forgiveness for his sins. A similar claim had already been made by Julian the Apostate. (25) We must also, I think, reject the suggestion of Guthrie (26) that the emperor acted in the interest of "dynastic legitimacy," that is, that he removed his illegitimate first-born son in order to secure the succession for his three legitimate younger sons. But Crispus must have committed, or at least must have been suspected of having committed, some especially shocking offense to earn him a sentence of death from his own father. He also suffered damnatio memoriae, (27) his honor was never restored, and history has not recorded the fate of his wife and his child (or children).16. Hieron. De Vir. Ill. 80; Euseb.-Hieron. Chron. Olymp. 276 (ed. Fotheringham 313; ed. Helm 231); Aur. Vict. Caes. 41.11.18. 188.8.131.52. 2.29.1-2.20. 13.2.38-41.21. Eutropius 10.6.3 (MGH, AA II 174; ad. Santini 67; ed. Friedhelm L. Müller [Stuttgart 19951 142); Aur. Vict. Epit. 41.11-12.22. Paschoud, Cinq études 37-38, and others before him.23. Hist. Franc., 1.34 in Migne, PL LXXI 179; 1.36 in MGH, SRM 1st ed. I 51; ibid. 2nd ed. I.1 26-27,24. 2.29.3-4.
Hans Pohlsander also gives us a concise bibliography for Crispus:
BibliographyRecent and concise accounts of the life and career of Crispus, with reference to the scattered primary sources, are the following:
A. H. M. Jones, J. R. Martindale, and J. Morris, The Prosopography of the Later Roman Empire I (Cambridge, 1971) 233.
Adolf Lippold in Der kleine Pauly V (1975) 1592.
Timothy D. Barnes, The New Empire of Diocletian and Constantine (Cambridge MA, 1982) 7-8, 44, and 83-84.
Ingemar König, Origo Constantini: Anonymus Valesianus (Trier, 1987) 136-38.
Dietmar Kienast, Römische Kaisertabelle, 2nd ed. (Darmstadt, 1996) 305-306.
The account by Otto Seeck, RE IV.2 (1901) 1722-24, has become in part obsolete.
More extensive or specialized studies are:
Josef Steinhausen, "Hieronymus und Laktanz in Trier," TZ 20 (1951) 126-54.
Patrick Guthrie, "The Execution of Crispus," Phoenix 20 (1966) 325-31.
Timothy D. Barnes, "Lactantius and Constantine," JRS 63 (1973) 29-46.
François Paschoud, Cinq études sur Zosime (Paris, 1975) 24-39.
Hans A. Pohlsander, "Crispus: Brilliant Career and Tragic End," Historia 33 (1984) 79-106.
Klaus Kremer, "Laktanz: Erzieher von Konstantins Sohn Crispus zu Trier," Kurtrierisches Jahrbuch 25 (1985) 35-59.
Samuel N. C. Lieu and Dominic Montserrat, edd., From Constantine to Julian: Pagan and Byzantine Views. A Source History (London 1996). Contains translation of and commentary on the Anonymus Valesianus, Origo Constantini.
Additionally references to Crispus will be found in all of the books on Constantine.
This gives us a good start for investigating the circumstances of Crispus' death in more detail. As we will see shortly, there is a lot more to say about this than a short summary can provide.