Either after the end of the Trojan War or the completion of the second Seven Against Thebes, Diomedes is recorded by some as returning to Calydon (from where his father Tydeus had been exiled only to perish fighting around Thebes with Eteokles and Polyneikes). Diomedes returns to his ancestral land to restore the throne to the line of Oeneus (which had been pushed out by Agrios). According to this tradition, Diomedes restores Andraimon, the father of Thoas (who appears in the Iliad) to the throne.
Euripides’ play Oeneus on this subject is lost, but we do have a passage where Diomedes’ arrives:
“Dearest field of my father’s land, Hail,
Kalydon, from where Tydeus fled the shedding of kin-blood
that son of Oineus, my own father
who settled at Argos and took as wife a child of Adrastos”
ΔΙΟΜ. ῏Ω γῆς πατρῴας χαῖρε φίλτατον πέδον
Καλυδῶνος, ἔνθεν αἷμα συγγενὲς φυγὼν
Τυδεύς, τόκος μὲν Οἰνέως, πατὴρ δ’ ἐμός,
ᾤκησεν ῎Αργος, παῖδα δ’ ᾿Αδράστου λαβὼν
This fragment doesn’t tell us much about the myth that we didn’t already know. But the story doesn’t go so well for elderly Oeneus. Diomedes takes him from Calydon to the Peloponnese where he is ambushed and killed by the surviving descendants of Agrios. The name Agrios—“wild one”—appears rather blandly in the Iliad. But outside that epic he is listed as the father of Thersites, thus a cousin of Diomedes.
According to the epic tradition, Achilles eventually kills Thersites and Diomedes makes the former go through a purification. Thersites is famous in book 2 for his destructive speech. Diomedes proves himself to be a capable speaker increasingly through the Iliad. And, his relative Thoas is quite good himself:
The use of Thoas here is intriguing. He is listed in the catalogue as the leader of the Aitolians (2.638) but he is also marked out for being exceptional in speech (Iliad 15.281-4):
“Then Thoas the son of Andraimon spoke among them.
Of the Aitolians he was the most knowledgeable with the spear
And best at running. But few Achaeans could surpass him in the assembly
Whenever the young men used to make a contest of words.”
Τοῖσι δ’ ἔπειτ’ ἀγόρευε Θόας ᾿Ανδραίμονος υἱός,
Αἰτωλῶν ὄχ’ ἄριστος ἐπιστάμενος μὲν ἄκοντι
ἐσθλὸς δ’ ἐν σταδίῃ• ἀγορῇ δέ ἑ παῦροι ᾿Αχαιῶν
νίκων, ὁππότε κοῦροι ἐρίσσειαν περὶ μύθων•