United as One recap

The final book turns the Garde's survival into an offensive. Working with human militaries and with the newly powered human Garde created when the sanctuary released Legacies into humanity, the surviving Loric go after the Mogadorian occupation city by city. Number Five surrenders and is imprisoned, offering intelligence on Setrakus Ra that the Garde need and John cannot forgive him enough to want. The obstacle is Setrakus Ra himself: nothing they have kills him, and the claim he made on Ella as his granddaughter has bound her life to his, so striking at him endangers her. John's Legacies expand until he can take on powers he watches other people use, and in the final assault he uses everything he has to destroy Setrakus Ra, dying in the act. The occupation collapses. Ella is freed, Six, Marina, Nine, Sam, Adam, and Malcolm survive, and a year later the survivors open an academy for the human teenagers whose Legacies the war produced.

Nine Loric children were sent to Earth with guardians called Cepans when the Mogadorians destroyed Lorien. Numbers One, Two, and Three were killed before the survivors ever met; every Cepan is dead, including Henri, who raised Number Four, John Smith. The survivors found each other one by one: Six, Seven (Marina), Eight, Nine, and Ten (Ella, from a second Loric ship), with John's human friend Sam Goode, Sam's father Malcolm, Sarah Hart, and the Chimaera Bernie Kosar. Number Five was found last and had already gone over to the Mogadorians; he killed Number Eight, and Marina took one of his eyes with a Legacy of lethal cold. Setrakus Ra captured Ella and told her she is his granddaughter and intended heir. Adam, a Mogadorian carrying the memories of Number One, defected and helped expose the enemy's plan to approach humanity openly. Setrakus Ra did so, then his fleet decloaked and the invasion began. In Mexico a surviving Loric presence released Legacies into ordinary humans worldwide; in New York Sarah Hart was killed, and John blames Five. Four of the nine are alive and free, one fights for the enemy, and Earth is occupied.

The Garde stop operating as fugitives and start operating as part of an alliance. Human governments and militaries, having watched their own forces fail against Mogadorian ships, accept Loric help, and the newly powered human teenagers appearing all over the world are being gathered rather than left to work it out alone. John leads without wanting to, grieving Sarah and increasingly unwilling to hear any plan that ends in anything except Setrakus Ra's death. Six, Marina, Nine, Adam, Sam, and Malcolm each take on a part of the effort, from training human Garde to reading the Mogadorian order of battle. The occupation itself is entrenched: the Mogadorians hold ground in cities across the planet and can reinforce from orbit, and no counterattack yet attempted has held anything for long.

Number Five gives himself up to the Garde. He is imprisoned rather than executed, over John's objections, because what he knows about Setrakus Ra, the fleet, and the Mogadorian command is not available anywhere else. Nobody in the group trusts a word of it, and everybody uses it. For John the arrangement is intolerable: Five killed Number Eight and John holds him responsible for Sarah, and he has to work with the intelligence anyway. Meanwhile Ella's connection to Setrakus Ra persists even at a distance, which makes her the Garde's best window into what he intends and a standing risk to every plan they make in her presence.

The alliance goes onto the offensive, retaking ground and hitting Mogadorian positions with a mix of human forces, Loric Garde, and the human Garde whose Legacies arrived weeks ago and who are being sent into combat with almost no training. The victories are real and expensive. Adam's knowledge of Mogadorian ships and command structures shapes the campaign, and Sam's ability to reach machines becomes a weapon in its own right. Throughout, the strategic problem stays unsolved: every Mogadorian position can be taken, and none of it matters while Setrakus Ra is alive, because he cannot be killed by anything the Garde have brought against him.

The Garde work out the shape of the problem they have been avoiding. Setrakus Ra survives injuries that would kill anything else, and the claim he made on Ella has left the two of them connected, so that harming him may kill her. Any plan that ends the war therefore has to solve both at once, and the only person developing fast enough to attempt it is John, whose Legacies have kept expanding until he can take on powers he watches other people use. He does not tell the others what that is likely to cost. The alliance commits to a final assault on Setrakus Ra himself rather than continuing to fight the occupation position by position.

The final assault reaches Setrakus Ra. The fighting costs the alliance heavily, and the Garde and human Garde together are able to bring the Mogadorian leader within reach but not to kill him. John, carrying more Legacies than any of them, breaks the link that protected him and destroys him, and dies doing it. Ella survives, released from the claim Setrakus Ra made on her. With their leader dead, the Mogadorian occupation collapses and the invasion ends. Six, Marina, Nine, Sam, Adam, and Malcolm survive, and so do the human Garde. What the survivors are left with is a planet full of people who now have Legacies, a war won at the cost of the person who held them together, and the question of what happens to the powers Lorien gave away.

The war ends with Setrakus Ra dead and John Smith dead with him. Ordinary force could not kill the Mogadorian leader, and the link he forged with Ella meant that any attempt risked killing her too. John, whose Legacies had grown to include taking on the powers he sees others use, is the one who finds a way through and spends his own life doing it. The Mogadorian occupation collapses without him. Ella survives, freed from the claim he made on her. Six, Marina, Nine, Sam, Adam, and Malcolm survive, along with the human Garde whose Legacies the sanctuary released. A year later the surviving Loric have turned the aftermath into an institution: an academy where human teenagers with Legacies are gathered, taught, and kept from being used as weapons by their own governments, run by the Garde who fought the war. The Loric survivors are no longer the last of anything, because the Legacies they carried have passed into humanity, and Earth ends the series with a generation of people who can do what the nine could and a peace that has to be managed rather than won.