Antilochus, the son of Nestor, brought Achilles the news that Patroclus had been killed. The scene appears in Book 18 of the Iliad, when the Greek forces are retreating and Achilles remains apart from the fighting because of his quarrel with Agamemnon.
Antilochus is chosen because he is closely connected to Achilles and is considered capable of delivering devastating news with compassion. When Achilles hears that Patroclus has fallen at Hector's hands, his grief becomes overwhelming. Antilochus holds Achilles' hands and stays with him because he fears Achilles may harm himself in his despair.
This message is the turning point that brings Achilles back into the war. His anger, previously directed at Agamemnon, is redirected toward Hector and the Trojans. The death of Patroclus therefore drives the final major movement of the Iliad: Achilles reconciles with Agamemnon, receives new armor made by Hephaestus, and returns to battle. Antilochus later dies fighting Memnon in the wider Trojan War tradition.