We learn of the fated death of Patroclus. How do we learn this before the events occur?

In Book XV, as the Achaeans drive the Trojans back behind the fortifications once again, Zeus wakes up and looks down on the battlefield with anger. He blames Hera, who responds that Poseidon helps the Achaeans of his own free will. Appeased, Zeus tells her his plan. Hector will reach the Achaean ships, and Patroclus will come out and fight. Patroclus will kill many great Trojans, including Sarpedon, Zeus's own son, but in the end Hector will kill Patroclus. Achilles will finally rejoin the battle to avenge his beloved companion.