the fourth book of Ember, The Diamond of Darkhold, Lina and Doon are curious when a roamer comes into town with a mysterious book. They trade a match for the book and discover it only contains eight pages. On the front of the book, printed in gold letters, it says, "For the People of Ember." Since they cannot make any sense of the book, they decide to go back to Ember. But when they return, the family of squatters have taken over the darkened city. The Troggs (Washton, Kanza, Minny, Yorick and an adopted boy called Scawgo) believing that they own Ember, rename it Darkhold. They capture Doon and Lina goes back to Sparks for help. Meanwhile, Lizzie (from Ember), Kenny, and Torren (both from Sparks) realize that the two are gone, and decide to go and try to find them. They don't succeed and a search party goes to look for them. While Doon is with the family they show him a diamond they found just outside Ember. Doon steals the diamond and escapes from Ember with help from Scawgo. While escaping, he also breaks the pipe connecting the generator to a waterwheel that created power for Ember, therefore shutting down Ember's lights for good. He finds Lina, who treats his broken ankle, and the search party finds them. Later, when Doon meets up with Lina they figure out that the diamonds are solar-powered sources of electricity. A lot of people decide to help them go back to Ember and look for other things to help in the winter. Doon looks for his bug book, and Lina looks for her city drawings. Neither of them are there. Lina stands on top of the mayor's building and says "Goodbye Ember, forever!" They bring back thousands of the new invention, which earns the town money and food to last the winter. A weary group of roamers, which are actually the Troggs, come into town one day, and a diamond is returned to them. But soon they learn the truth. There are hundreds of diamonds, thousands, and they can power all sorts of electronic devices. They also learn that Doon and many others lived in Ember, and that they left and evacuated because Ember was dying. So the Troggs come to live in Sparks as well. And in the end, it is revealed that in the future, cities are rebuilt with the power of the diamonds and Lina, Doon, and Poppy all live together in a house in Sparks.