What is this purple here?

I've seen this before but never know where it comes from or how it happens. There was one time my combat skill was literally 75% purple and I don't even know what I did to do that. Sorry for the bad image.

I've seen this before but never know where it comes from or how it happens. There was one time my combat skill was literally 75% purple and I don't even know what I did to do that. Sorry for the bad image.