This naming structure is characteristic of unauthorized distribution.
x264 . This is a popular library for encoding video streams into the H.264/MPEG-4 AVC format, known for high compression efficiency while maintaining visual quality.
MKV . A flexible, open-standard container format that can hold unlimited video, audio, picture, or subtitle tracks in one file. Source Attribution
This is likely a "release tag" or a watermark from a specific content distribution site or pirate group. These groups often append their site name to the beginning of files they upload or re-encode. Safety & Security Warning
Files with this naming convention—specifically those found on third-party distribution sites—carry several risks: