[1.2.381] The Wind

That buff comes from the Saint Saver event, the purple crystal on the side of the screen. I don't know how it works or what it gives though :S

Every so often, a little bar pops up, and once you hit a monster 500 times, and I'm positive multihits skills only count as one hit, you can press attack and activate the buff.

 Spoiler

Well, I see 공격력 and 40, so I assume it's a 40 attack buff... or it's an attack buff that lasts 40 minutes, but I don't know Korean, so yeah.