Whistleblower is DLC for Outlast and acts as a prologue set just before the events of the original game as well as an epilogue. After the introduction the story soon runs parallel with the main game. The player takes on the role of software engineer Waylon Park, the man who tips off Miles Upshur, the protagonist of the main game, about the events in the Mount Massive Asylum. Park works at the asylum for Murkoff Corporation working directly with the Morphogenic Engine for the experimental treatment of patients and he witnesses the torture that Billy Hope endures. After sending an e-mail to Upshur informing him, he is caught by his employed Jeremy Blaire and forced to endure the Morphogenic Engine himself for two hours as a test. However, as the supernatural entity Walriders escapes and causes mayhem at the institution, Park manages to escape. His goal is to escape the facility and inform the authorities.
The game reuses some assets of the original game's environment, but the journey is in a reversed order. Park starts in the underground laboratories and needs to make his way up to the asylum. Next to the existing cast, the game introduces the new characters Jeremy Blaire (Park's employer) and the patients Eddie Gluskin and Frank Antonio Manera. The gameplay is identical, with a focus on stealth and navigating the environment to avoid encounters. The horror elements with gruesome scenes remain present and Park too uses a camcorder that provides night vision. Just like the original game he is powerless against ghouls and needs to flee when spotted. There are generally less encounters with patients compared to the original game. Various conversations and notes provide background to the story.