Osiris is the ancient Egyptian god of the dead—but he wasn't always. When Sadie has a vision in the Hall of Ages, she sees Osiris while he's still alive (in whatever sense gods are alive). He's "a muscular African man in rich black robes. He had a handsome face and warm brown eyes. His hands looked strong enough to crush rocks" (15.18). In this incarnation, he's king of the gods, and he's quite powerful.
In an earlier incarnation, Set tricked Osiris into getting inside a magic sarcophagus that trapped him. Isis and Horus find the box, but Set shatters it, so that the pieces of Osiris's body are scattered to the winds.
Horus narrates the story to Carter: "My mother and Aunt Nephthys spent years searching for the pieces of the coffin and Father's body. When they collected all fourteen, my cousin Anubis helped bind my father back together with mummy wrappings, but still Mother's magic could not bring him back to life fully. Osiris became an undead god, a half-living shadow of my father, fit to rule only in the Duat" (26.137).
The way Horus tells it, Osiris was demoted from a powerful god to a less powerful version of himself. Osiris sees it differently, though. At the end of the book, Julius—who is also Osiris—tells Carter that when Osiris was alive, he was a great king, but when he died, he became a thousand times more powerful. So that's another way of looking at death, huh?
Osiris has to die in order to be resurrected as god of the dead. This is a fact. So when Julius deliberately merges his soul with Osiris's, the primary goal is not, as the kids first guessed, to try to resurrect their mom, though that would be cool. As Julius tells Sadie near the end of the book: "Osiris must take his throne… Through death, life. It is the only way" (39.82).
Due to Osiris's nature, Julius was planning all along to sacrifice himself as Osiris's mortal host in order to give Osiris the boost he needed to return to the throne of the Hall of Judgment. It's a little weird when phrased like that, but Julius had already lost the love of his life, so why not go along with a suicidal plan to restore the order of the universe? That's his thinking, anyway.
In fact, Set's plan to entomb Osiris within the red pyramid throws a wrench into the whole resurrection thing. As Amos summarizes it to the kids: "The coffin is inside the pyramid. They're planning to use Osiris's power to augment the storm. When Set unleashes it at sunrise—and it will be quite an explosion—Osiris and your father will be obliterated. Osiris will be exiled so deep into the Duat he may never rise again" (31.15). This, folks, is bad news: it's not a new version of the old pattern, but a disruption of it that will screw up the whole world order.