This entry isn't about a single prophecy, but rather a series of individual prophecies covering judgments, war, famine, plagues and other events that will happen both during the Tribulation and the Pre-Tribulation period, starting from approximately around the time of the Rapture forward to the second coming in which a massive amount of death and world depopulation occurs. At the start of this period the world population will potentially be as high as 7.5 billion. By the time of the second coming, when this will all end, there will be less than 500 million people left on Earth, possibly as few as a hundred million, a reduction of up to 98% of the Earth's population that existed at the time of the rapture. But at the absolute most, no more than 10% of those alive at the time of the rapture will still remain at the end.
EDIT (2019-08-09): I'm marking this prophecy as "in progress" due to recent events moving us into, at least what I believe is the first steps in the direction of global depopulation. I still don't believe the main bulk of it will happen until AFTER the rapture, but I do think that we'll see, at the very least, some small opening volleys in this prophecy's fulfillment. Just the size of the proofs section for this prophecy alone should give you plenty of reasons to believe that to be true.
(*Do you have a better description, or did you find an error? Please let me know.