The Suda Has the following Entry:
Diomedean Compulsion: “This is also called a horse; a proverb from either the son of Tydeus or from the Thracian Diomedes who compelled guests to sleep with daughters who were ugly (and whom some allegorize as horses), or he would kill them.
And some say that Odysseus and Diomedes, after stealing the Palladion, returned during the night. Odysseus, who was following, planned to kill Diomedes. But when Diomedes saw the shadow of the sword in the moonlight, because he feared Odysseus, he made him walk in front of him, slapping him with the sword in the middle of the back. This proverb is used when people do things under compulsion.
For this reason, Diomedes kept man-eating horses: in the departure he was greatly aggrieved and was not welcomed to his own home, but after he was exiled he went to Kalabria and founded a city which he called Argurippê but whose name later was changed to Benebentos.”
Διομήδειος ἀνάγκη. λέγεται καὶ ἵππος. παροιμία, ἀπὸ τοῦ Τυδέως ἢ ἀπὸ τοῦ Θρᾳκός· ὃς ἠνάγκαζε τοὺς ξένους αἰσχραῖς οὔσαις ταῖς θυγατράσιν αὐτοῦ μίσγεσθαι (ἃς καὶ ἵππους ἀλληγορεῖ), εἶτα ἀν-ῄρει. οἱ δέ, ὅτι Διομήδης καὶ ᾿Οδυσσεὺς τὸ Παλλάδιον κλέψαντες νυκτὸς ἐπανῄεσαν. ἑπόμενος δὲ ὁ ᾿Οδυσσεὺς τὸν Διομήδην ἐβουλήθη ἀποκτεῖναι. ἐν τῇ σελήνῃ δὲ ἰδὼν τὴν σκιὰν τοῦ ξίφους ὁ Διομήδης, δείσας τὸν ᾿Οδυσσέα ἐποίησε προάγειν παίων αὐτοῦ τῷ ξίφει τὸ
μετάφρενον. τάττεται δὲ ἐπὶ τῶν κατ’ ἀνάγκην τι πραττόντων. διὰ τοῦτο λέγει, ὅτι ἵππους ἀνθρωποφάγους εἶχεν ὁ Διομήδης. ὅτι Διομήδης εἰς τὸν ἀπόπλουν καταχθεὶς εἰς τὰ ἴδια οὐκ ἐδέχθη, ἀλλὰ διωχθεὶς ἀπῆλθεν εἰς Καλαβρίαν καὶ κτίζει πόλιν, ἣν ἐκάλεσεν ᾿Αργυρίππην, τὴν μετονομασθεῖσαν Βενεβεντόν.
Hesychios the Lexicographer discusses the same two origins for the phrase:
“Diomedean Necessity: A proverb. Klearkhos says that Diomedes’ daughters were absolutely wretched and that some were forced to sleep with them or he murdered them immediately. In the little Iliad, the story is that the phrase comes from the theft of the Palladion.
Διομήδειος ἀνάγκη· παροιμία. Κλέαρχος μέν φησι, Διομήδους
θυγατέρας γενέσθαι πάνυ μοχθηράς, αἷς ἀναγκάζειν πλησιάζειν
τινάς, καὶ εὐθὺς αὐτοὺς φονεύειν· ὁ δὲ τὴν μικρὰν ᾿Ιλιάδα
φησὶ (fr. 9 Allen) ἐπὶ τῆς τοῦ Παλλαδίου κλοπῆς γενέσθαι
There is one fragment from the Little Iliad about this moment:
“It was the middle of the night, and the bright moon lay on them”
νὺξ μὲν ἔην μεσάτη, λαμπρὴ δ’ ἐπέτελλε σελήνη.
This, admittedly, doesn’t say much. The basic story is that, in order to take Troy, the Greeks needed to steal the Palladion, an image of Athena. Odysseus and Diomedes sneaked into the city to get it. On the way back, Odysseus tried to kill Diomedes. According to the fragments of the historian Konon, Diomedes climbed on Odysseus’ shoulders to get into the city, but then left him behind to secure the Palladion himself. According to other accounts (summarized by Servius in his commentary on the Aeneid, see Gantz 1992, 643-5), Odysseus just wanted the glory all to himself.
In any case, the Palladion-tale is a re-doubling of other Trojan War Motifs: the requirement of Herakles’ bow and Philoktetes or the need to have Neoptolemus present, for example, are similar talismanic possessions to end the long war. Odysseus’ conflict with Diomedes, here, is not dissimilar either to his quarrel with Ajax or his feud with Achilles (mentioned in the Odyssey). This narrative, also engages with the pairing of Diomedes and Odysseus elsewhere, especially Iliad 10.