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You decide that Sirius Black's superior moral strength is the stronger argument, and you persuade the Potters to stick with their original plan to let you make Sirius Black their Secret Keeper. Sure enough, Voldemort immediately confronts Sirius and demands to be told the Potters' location, threatening to kill Sirius if he refuses. Sirius refuses. Voldemort casts the Imperius Curse on Sirius. Sirius resists it. Voldemort casts the Cruciatus Curse on Sirius. Sirius continues to resist until he passes out. In a rage, Voldemort casts the Avada Kedavra Curse on Sirius, making the Potter's secret unattainable.
Realizing that he now has no way of finding the Potters, Voldemort decides to go after Neville Longbottom. He barges into the Longbottoms' house and finds himself facing two armed, prepared and proficient fighters. He manages to kill Frank Longbottom, but is in turn killed by Alice Longbottom.
Voldemort has long been prepared for such a contingency. He has created a number of Horcruxes - pieces of his soul - so that instead of dying along with his body, Voldemort is turned into a spirit. To become a person again, he needs one of his Death Eaters to find him, create a rudimentary body for his spirit to inhabit, and then prepare a potion in which to immerse him. But, since he doesn't trust his Death Eaters, the only one who knows how to find him is his newly recruited spy Peter Pettigrew. Peter entered Voldemort's service out of fear, and now that Voldemort has become less powerful than his enemies, Peter once again switches sides; so Voldemort is doomed to remain a spirit forever.