The story that Constantine killed Crispus via 'cold poison' is only preserved in a letter by Sidonius Apollinaris, a Gallic noble from the fifth century.

It seems to me that no greater power of satiric suggestion was shown by the consul Ablabius when in a couple of verses he punctured the life and family of Constantine and bit them with this distich posted up secretly on the doors of the palace!

"Who would now want the golden age of Saturn? Ours is a diamond age – of Nero’s pattern."

<He wrote this> of course because the aforesaid Augustus had almost simultaneously got rid of his wife Fausta with a hot bath and his son Crispus with cold poison.

As the source was relatively late and Sidonius was clearly trying to make a rhetorical point, I don't buy that Crispus was killed by 'cold poison'. Fausta's death in a bath is well-attested to though, but even so there are various suggestions - that it was an abortion gone wrong, a suicide or that she was killed on the orders of Constantine.

Other, much earlier sources only claim that Crispus was 'most cruelly killed' (Jerome), whilst Zosimus and the Epitome de Caesaribus added the fact that he was allegedly executed for adultery with his step-mother Fausta. Note that the common source of both accounts was the pagan Eunapius of Sardis, who was not a big fan of Constantine. Zosimus was a pagan too and we don't know the author or the context of the Epitome, so we shouldn't take these two sources at face value. An even earlier source, Aurelius Victor, wrote only that:

[...] the eldest of these had died on the orders of his father, the reason is uncertain

This indicates a trial of some sort, but not much more than that. Based on my reading, Crispus was probably quietly executed for treason at the instigation of Fausta, who wanted to promote the interests of her sons over her step-son. Over time the story was distorted, since no matter what the truth was, the deaths of Crispus and Fausta were clearly scandalous and made for a good story at the hands of Constantine's many enemies.

You know, I specifically remember reading about Crispus being ordered to kill himself with poisons, but for the life of me I can't find the primary sources for that particular tale. Here's what I've been able to dredge up about Crispus' death:

• "But Constantine, when mastery of the entire Roman empire had been obtained through the wondrous good fortune of his wars, with his wife, Fausta, inciting him, so men think, ordered his son Crispus put to death. Then, when his mother, Helena, as a result of excessive grief for her grandson, chastised him, he killed his own wife, Fausta, who was thrown into hot baths" (Epit. Caes. 41.11)

• "At this time the Roman empire fell under the sway of one emperor and three Caesars, a state of things which had never existed before; the sons of Constantine ruling over Gaul, the east, and Italy. But the pride of prosperity caused Constantine greatly to depart from his former agreeable mildness of temper. Falling first upon his own relatives, he put to death his son, an excellent man; his sister's son, a youth of amiable disposition; soon afterwards his wife, and subsequently many of his friends" (Eutr. *Brev.*10.6).

• "But when he came to Rome, he was filled with pride and arrogance. He resolved to begin his impious actions at home. For he put to death his son Crispus, styled (as I mentioned) Caesar, on suspicion of debauching his mother-in-law Fausta, without any regard to the ties of nature" (Zos. NH 2.29).

Meanwhile, Sozomen and the Emperor Julian both mention Crispus' death, when discussing Constantine's remorse (Jul. Caes. 336, and Soz. HE 1.5).

Unfortunately Crispus is a bit of a muddled figure in history. Origo Const. doesn't mention Crispus' death at all, only praises him for his military prowess in the war against Licinius. And Aurelius Victor doesn't mention Crispus at all. Neither does Eusebius in his Vita Constantini (the sycophantic scamp!)

Thanks to shlin28 & DasImp for the Sidonius Apollinaris references. I looked up Epist. 5.8.2 here: http://www.sidoniusapollinaris.nl/ and this is what it says:

quia scilicet praedictus Augustus isdem fere temporibus extinxerat coniugem Faustam calore balnei, filium Crispum frigore veneni.

shlin28's comment above translates that like this:

of course because the aforesaid Augustus had almost simultaneously got rid of his wife Fausta with a hot bath and his son Crispus with cold poison.

'frigore veneni” is translated as “cold poison”.

I took Latin but I’m not a Latin scholar. I looked up the 4 Latin words, calore, balnei, frigore & veneni, in Whittaker’s Latin-English dictionary here: http://ablemedia.com/ctcweb/showcase/wordsonline.html

All 4 of those words are nouns.

Calore is the ablative form of calor, caloris (heat).

Balnei is the genitive form of balneum, balnei (bath).

Frigore is the ablative form of frigor, frigoris & of frigus, frigoris (cold or chill).

Veneni is the genitive form of venenum, veneni (poison).

More literal translations might be “by the heat of the bath” and “by the cold of poison” or “by the chill of poison”.

Sometimes fevers can be accompanied by chills and shivering. And some types of poisoning, such as food poisoning, can produce chills. Maybe he’s referring to something like that and not to any particular type of poison. I'm not saying Crispus died of poison, just wondering what the sources mean when they say that. They all get that statement from that one guy, Sidonius Appollinaris.