What is this???? Weird filter on splash art and a chest with a + ????

It's for the Draven Day event; Legendary Spinning Axes.

If you receive a Legendary (by any means) you'll get a free Mystery Skin with it. If that is Legendary too it keeps going. Any skin with the chest icon is eligible (they have the icon just in case you don't know which ones are Legendary).

http://na.leagueoflegends.com/en/featured/skins/draven-day-2016#section-axe