Do you have the file size or any specific error messages when trying to open this archive? Support for rar v5 files? - ExifTool by Phil Harvey
Open the .rar file in a hex editor. A standard RAR5 file should start with the hex signature 52 61 72 21 1A 07 01 00 . Sigma5 Files.rar
"Sigma" is a generic open signature format for SIEM systems. This could be a bulk collection of threat hunting rules. Do you have the file size or any