This is a fan-made campaign created many years ago. Because it is older community content, it is generally available for free as it was never a commercial product.
The most reputable place to find legacy Age of Empires II mods and campaigns is Age of Kings Heaven (AOKH) . It is a long-standing community archive.
Drop the downloaded campaign file into that folder. The Trasamire Campaigns Free Download
Here is the "report" on where to find it and how to stay safe:
Avoid any site that asks you to complete a "survey," download a separate "installer" (.exe), or provide a phone number to unlock the file. These are common signs of malware or phishing. This is a fan-made campaign created many years ago
Find your game installation directory (e.g., C:\Program Files\Microsoft Games\Age of Empires II\Campaigns ).
Authentic campaign files for Age of Empires II usually end in extensions like .cpn , .scx , or .scn . If you download a ".zip" file, it should only contain these file types and perhaps a text readme. Installation (General Guide) It is a long-standing community archive
Searching for a free download of typically points toward a specific campaign or module for the classic real-time strategy game Age of Empires II (likely the Age of Kings or The Conquerors editions).