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ADD BRAINLIEST
A story is told through a log, diary, or journal that a character used to document their activities and progress through the backstory before something bad happened to its writer. A staple of Post-Apocalyptic Fiction to explain to the cast how the world ended up the way it is, it can also be used as a handy way to fill in heroes who arrive Late to the Tragedy, and one of the first (or last) things that a "rescue" party answering a Distress Call will find.
You can expect that whoever made this log will have recorded events up until their last breath. If the heroes (and thus the audience) choose to read this log from the beginning, the log will no doubt start and progress the same way, with hopeful characters recording the casual details of their lives and work, until things start going sideways and the entries shift towards concern, disbelief, desperation and/or (ultimately) insanity. The final — and usually the most important — entry may range from incoherent gibberish as the remaining character tries to warn the world of what happened to them, to a final cogent statement warning the reader not to repeat their mistake (or how to otherwise clean up after them).
This log can be written or recorded in a variety of ways; if it's a video log, the downhill progress of the situation will be punctuated visually, e.g. degradation in the appearance of the character, their surroundings, or even the video itself. Bonus points if a video log's final entry shows the character ultimately succumbing to whatever horror took over. Grand prize if it's a written log that somehow still records the author's final dying moments.
Depending on the timeframe and nature of the apocalyptic event, the log may have gone through Ragnarök Proofing in order to be legibly retrieved by the heroes.
See also Lost in Transmission, Distress Call, Late to the Tragedy, Action Survivor, Almost Dead Guy, Harbinger of Impending Doom, Send in the Search Team, Ignored Expert, Undead Author, Posthumous Character, Posthumous Narration, That Was the Last Entry. See also Video Will, the various times when the Cassette Craze applies to disappearances, and some of the less pleasant cases of Message in a Bottle. Deadline News is a related trope where the very bad thing happens to the TV news crew reporting live on it; Unintentional Final Message is another related trope where the doomed sender doesn't know they're about to die.
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