Files like these are and not official releases from Epic Games. It is standard practice to: Scan the .rar file for malware using tools like VirusTotal .
The specific version of Unreal Engine 4 files required to run that build. Fortnite 3.5 reupload by blk.rar
This specific file is used to play on private servers such as , Project Era , or Polaris . These projects allow players to revisit the Chapter 1, Season 3 map and gameplay mechanics by redirecting the client's connection from official Epic Games servers to a private hosting environment. Safety Note Files like these are and not official releases