E. Jaya and Vijaya
There is a widely told story that explains Ravana as something other than a rakshasa king born to a sage and a rakshasi. In it he is a doorkeeper of heaven, serving out a curse. Readers who have met it sometimes arrive expecting this book to contain it. It does not, and this section is here to say where it comes from instead of leaving a silence.
The story
Jaya and Vijaya guard the gate of Vaikuntha, Vishnu's own realm. Four sages, the Kumaras, come to see Vishnu and are turned away at the door. Being sages of great power, they curse the two gatekeepers to fall out of Vaikuntha and be born in the world below.
Vishnu does not overturn the curse. He offers terms. The two may live many lives as his devotees and return slowly, or a few lives as his enemies and return quickly. They choose the shorter road, which is also the harder one: three births, each ending with Vishnu killing them.
The three pairs are Hiranyaksha and Hiranyakashipu, then Ravana and Kumbhakarna, then Shishupala and Dantavakra. On this account, the war in this book is the second of three appointments, and Rama, who is Vishnu, is keeping it.
Where it comes from
The Bhagavata Purana. The curse is told in its third book and recapped in the seventh, which is where the three pairs are named together as a set. Different translations divide and number the chapters differently, so a chapter reference that works in one edition may not in another. The exact verse in which the choice between many devotee lives and few enemy lives is offered is one this project could not confirm directly, so no verse number is given here.
The Bhagavata Purana is a much later text. Modern scholarship generally places its composition somewhere between roughly 500 and 1000 CE, with many scholars favoring the ninth or tenth century for the form it now has. Valmiki's Ramayana is older by a long way: its core is usually dated to the middle of the first millennium BCE, with later material added over following centuries. Even the latest layers of the Ramayana, which include the Uttara Kanda that supplies much of this book's second half, are earlier than the Bhagavata Purana's final form.
It is also worth being exact about scope. This is a Vaishnava telling, centered on Vishnu. It is not a claim the whole of Hindu tradition makes, and this project found no parallel version of it in the Vishnu Purana, contrary to what is sometimes said.
What Valmiki gives instead
Chapters 51 and 52 of this book. Ravana and Kumbhakarna are the sons of the sage Vishrava, son of Pulastya, and Kaikasi, daughter of the rakshasa Sumali, who was sent to Vishrava by her father to recover through her children what her line had lost. Their brother is Vibhishana and their sister is Shurpanakha. Ravana's power comes from his own austerities and the boon he wins with them, and the flaw in that boon is his own contempt: he does not trouble to ask for protection from human beings.
There is no earlier curse in that account and no assigned role. He does what he does.
What the two accounts are each for
It would be easy to treat the Jaya and Vijaya frame as the real explanation, the machinery behind the story, and Valmiki as the surface. That is not how to read it. They are different texts from different centuries doing different work.
The Puranic frame answers a question Valmiki does not ask: how can enmity toward God be part of God's own order? Its answer is that opposition can itself be a form of relation, and Vaishnava tradition has terms for this, describing the three enemy lives as a devotion turned inside out. Read that way, Ravana is not damned. He is going home the fast way, and his death is his release. This is the tradition's own theological reading of its own text, not a finding about history.
Valmiki's account answers something else, and this book follows it: what a person with that much power and that much learning does with a life that is entirely his own. The boon that fails him fails because of what he thought of human beings. Nothing arranged that in advance.
Neither account cancels the other, and neither is a corrected version of the other. They simply cannot both be held at the same moment while reading, which is why this one is here at the back and not in the story.