Devotio: the ritual dedication of an enemy, or self-dedication, to the gods of the Underworld. This Roman custom is also known from other ancient nations.
The most famous devotio in Roman history is probably that of the consul Publius Decius Mus, who was fighting a battle against a formidable coalition of Samnites and Gauls at Sentinum in 295 BCE. According to Livy,note[Livy, History of Rome since its Foundation 10.17ff.] he dedicated himself to the gods of the Underworld and rode, on horseback, to his enemies, who killed him. The wrath of the gods now was upon the Gauls, who were soundly defeated, after which it was easy to break Samnite resistance too.
Of course Livy's story may be propaganda - the man was killed in action and it was later claimed that this was not just bad luck, which might demoralize the soldiers, but a pious and inspiring act of patriotism. However, there are too many stories about devotio to deny that it was a ritual that had some sort of reality, even when it did not belong to the most important cult practices of ancient Rome. Livy claims that Decius Mus' father devoted himself before the battle of the Vesuvius in 340 VC (= 336/335 BCE). His account includes the prayer that was spoken, although is is probably not the authentic version: "I devote the army and auxilaries of the enemy and myself to the Di Manes and Tellus".note[Livy, History of Rome since its Foundation 8.9-10.] The son of the man who died at Sentium was also killed after a devotio, according to Cicero, in the battle of Ausculum, in which king Pyrrhus of Epirus was ultimately victorious.note[Cicero, Questions debated at Tusculum, 1.89; The Ends of Goods and Evils, 2.61.]
A fourth case is referred to by Livy in his account of the sack of Rome in 390 VC (=387/386 BCE): several old senators were sitting in their most beautiful togas in their houses, waiting to be killed by the Gauls.note[Livy, History of Rome since its Foundation 5.41.3-10.] A final example is the self-sacrifice of Marcus Curtius in 362 VC,note[Livy, History of Rome since its Foundation 7.6.1-6.] which is connected with the monument known as Lacus Curtius.
The ritual itself was simple. The pontifex maximus said the prayer, and the general who dedicated himself repeated it, leaning on a spear, and dressed in a toga. With the toga over his head ("Gabine fashion"), the commander rode to the enemy. If he survived, he was never to perform religious acts any more; if an ordinary soldier had dedicated himself to the Underworld and had survived, a statue with a height of seven feet had to be buried instead. One such statue has been excavated in the country of the ancient Vestini, at Capestrano in the Abruzzi.
When Rome became a monarchy, the word devotio was used to describe the self-sacrifices for the well-being of the emperor. A notorious example is the story of Publius Afranius Potitus, who promised to commit suicide if only the emperor Caligula would recover from an illness - the emperor insisted that the man would indeed descend to the Underworld.note[Cassius Dio, Roman History, 59.8.3; cf. Suetonius, Caligula, 27.2.]
A Greek Ritual?
Although devotio is an Italian ritual, the idea that a soldier could die for his comrades is also attested in Greek legends. In the first place, there's the story of the Athenian king Codrus.note[Lycurgus, Against Leocrates, 84-87.] Euripides tells how the Theban crown prince Menoeceus sacrificed himself during the siege by the Seven against Thebes.note[Euripides, Phoenician Women 911ff; cf. Pausanias, Guide to Greece, 9.25.1.] That self-sacrifice was also a reality at the battlefield, is proved by the seer mentioned by Xenophon.note[Xenophon, Hellenica, 2.4.18-19.] Perhaps the famous oracle given to Leonidas before the Battle of Thermopylae, that Sparta would either be sacked or regret the death of its king, can be interpreted in this fashion.
Devotio is very likely to be behind Herodotus' story of the Battle of Himera, in which the Carthaginian general Hamilcar throwing himself in the fire.note[Herodotus, Histories, 7.165-167.]