The Achaians are suffering from plague; something has to be done.
Not counting the brief scene of Chryses's embassy, this is the opening situation of the first major scene in the Iliad, which sets the stage for all future conflict.
After his falling out with Agamemnon, Achilleus refuses to fight and the Achaians start getting killed. Not even the offer of gifts can turn him around.
You can think about pretty much everything from the beginning of the book up to Book 16 as part of the conflict stage, because it all emerges directly out of (and is the fulfillment of) Achilleus's prayers in Book 1.
Nestor suggests that Patroklos lead the Myrmidons, wearing Achilleus's armor. He does so and is killed.
Up until now, everything has been going according to Achilleus's plan. At this point, though, things go majorly off the rails.
Achilleus gets new armor, comes back to battle, kills lots of Trojans, then kills Hektor.
This moment marks the high point of the chain of actions initiated by Achilleus's refusal to fight for the Achaians. Everything that follows is due solely to Achilleus's own emotional state.
Achilleus begins abusing the body of Hektor, which the gods will not permit to decay or be destroyed.
At this point, we are in a whole new ballgame, and we (and the gods) have no idea what Achilleus is going to do next—or when he will come back to sanity.
The gods take pity on Hektor and send Achilleus's mother Thetis to instruct him to give the body back. They also tell Priam to go ask for it. Priam goes to Achilleus's tent and they connect with one another as human beings, share a meal, and negotiate the surrender of the body.
This scene marks the true end of Achilleus's inhuman wrath. By sharing a meal with Priam, he marks his re-entry into the world of ordinary human relations.
The funeral of Hektor.
By showing us the reactions of Hektor's loved ones back in Troy, the poet lets us share in the emotional connection forged between Achilleus and Priam. This scene provides a vivid picture of the humanity of the Trojans, which we know will be violated when the city falls. At the same time, the image of the hero's funeral looks forward to the funeral of Achilleus, which we know will be coming from the constant foreshadowing of his death.