The night raid (Doloneia)

The night between two days, neither side sleeping. Agamemnon walks the camp in his cloak, unable to rest. Nestor, also awake, proposes a reconnaissance: send a man into the Trojan camp to learn what they're planning. Diomedes volunteers. He picks Odysseus to go with him. The two arm themselves with simpler gear suitable for the dark — leather caps rather than bronze helmets, no shields — and slip out into no-man's-land.

On the Trojan side, Hector has had the same idea. He calls for a volunteer to spy on the Greeks; a young aristocrat named Dolon offers, asks for Achilles's horses as a reward when the war is won, and goes out into the dark in a wolfskin cap. Diomedes and Odysseus see him coming, hide behind corpses, ambush him. Dolon falls to his knees and begs. They interrogate him. He tells them the Trojan dispositions — and, fatefully, that a Thracian king named Rhesus has arrived that day at the edge of the camp with magnificent white horses. The Greeks listen, then kill him.

They go on. They find Rhesus's camp. The Thracians are asleep. Diomedes kills twelve men and Rhesus himself, while Odysseus drags the bodies aside so the horses can be led out without trampling them. They take the white horses and ride back to the Greek camp through the dark, arriving before dawn. The book is famously isolated from the rest of the poem — the white horses are never mentioned again, the raid changes nothing. Some scholars have argued it was added later. It is in the poem, it is gripping, and it does what it does.