"Rune" skill?

Sometime, i see others sor cast this. Is it weapon buff or skill? @@
Attachment 59080

Its the Proc of the Mythic Gun.

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I use it all the time and still amazed by the others who either ask what it is or think it is a part of the game and try to attack it!....lola

I remember when this first came out... I'd sit at Pier and cast it all over and watch the noobs run around hacking at it! hahahahaha