User blog comment:Mckrongs/Monster Hunter 4 for the Nintendo 3DS/@comment-176.44.90.243-20120908171150/@comment-5489419-20120908190041

I have, too, but I was thinking that it was just a full Spirit Blade charge, since in MH3 it made the blade glow red, as well. Maybe it's just a special Longsword, though!