This prophecy unfolds on the heals, and likely some months, after the end of the fourth trumpet judgment. To set the picture I begin with a reminder of what the Earth is like at this point. So far it's been ripped apart by war, famine, plague, a mega earthquake, a gigantic asteroid impacting the sea and destroying 1/3rd of all sea life, a second asteroid that has poisoned 1/3rd of all drinkable water on the Earth, and a plague of cold and darkness that spans a period of up to 4 months or more. So basically by this point Earth has become a barren, desolate wasteland filled with starving people, destroyed cities, decimated nations, ravaged landscapes and more.
Essentially it's become Hell on Earth at this point and we're not even to the midpoint of the Tribulation yet which begins when the Beast (the AntiChrist) desecrates the third temple. So this judgment occurs no less than six months prior to that event as the prophecy itself takes five months to unfold, and the remaining two trumpets and the events which follow can easily unfold within a matter of days. Those who are saved at the time of this judgment won't be affected by it or the two to follow simply because these judgments are only for the unsaved to punish them for their sins, and to, despite seeming unusual and excessive, convince them of their wrong and to encourage them to seek God. Sadly, they won't, but that doesn't mean God won't try, even with something this extreme.
This judgment begins with a star (this time it's an angel rather than an asteroid) who is given a key (ie, permission) to release a hoard of demons, in the form of locusts from the pits of Hell, that will roam the Earth for a period of 5 months (whether they die afterwards or simply return to Hell is unknown at this time, only that they suddenly go away at some point) attacking and stinging and hurting men and women all over the globe. The bite they administer is described as being like a scorpion sting causing very excruciating pain.
The most interesting part about this is, anyone who is bit by these locusts will immediately become immortal and unable to die, no matter what they do. This will likely be because God will supernaturally make every man and women who is alive on the planet at this time completely immortal and invincible and incapable of dying. As such you could literally explode a 500lb bomb in someone's face and they would walk away without a scratch. That won't be the case once the 5 months is over, but it will be during this time. It's so that people who are stung are forced to face the full torment and agony of the sting, unable to escape it through death, which of course, if they're unsaved would only make their situation worse. Even so, this judgment isn't meant as punishment so much as it's once again God's attempt to win the lost by giving them a small taste of Hell to help them understand that if they don't get saved, they'll be facing far worse pain than this.
Sadly, despite all God does to get the attention of these stubborn, stiff necked people, they refuse to turn to Him. Some might say, "Well yeah, neither would I!" But it's not because of what God's doing, but rather because they've already determined in their hearts that they will not turn to God nor listen to Him nor turn from their evil and wicked ways for any reason whatsoever. On an up note, due to the intensity of this plague and it's incredibly debilitating effects, those who are saved, born again believers at this time will actually experience a brief period of peace as their enemies will be temporarily incapacitated and unable to persecute them until the plague runs its course. One final reminder, this judgment is targeted specifically at unbelievers and will not harm anyone who is saved at that time as they will be under God's protection, and thus these locusts will be unable to attack or harm them.
(*Do you have a better description, or did you find an error? Please let me know.