My world's like that. Dinah is, unfortunately for everyone especially her, extremely reliable. She predicts the future in probabilities. 80 percent chance the depopulation begins in two years, 100 percent chance by 15. Whenever it begins, 100 percent chance it's complete inside a year. Everyone doesn't die at once, but it begins with a massive die-off, and then the numbers trickle down to zero in drops and jumps. If we'd killed Jack Slash, that 80 chance would have moved to 8 years away, but the 15 year date didn't change. But we don't know what questions to ask to get who, or even what is the actual cause. Got lots of ideas - we have three giant monsters called Endbringers mooching about, after all - but nothing certain.