The Harrowing Across Valoran
http://forums.na.leagueoflegends.com/board/showthread.php?p=3452447#post3452447
Each calendar year, the celestial bodies of Runeterra align in a particular astronomical event. This lengthy occasion - beginning with a partial eclipse on the 18th of October and culminating in a full solar eclipse on the 31st of October - is of one of the most significant holidays in Valoran. The alignment occurs annually around the time of the harvest, and is known as the Harrowing due to its dusky setting. For obvious reasons, the occasion is marked by superstitions surrounding the world's dark denizens, creepy crawlies, and things that generally go bump in the night.
While rural folks still believe that this is a time of great foreboding, most of Valoran has taken to commemorating the Harrowing through a bit of good, clean fun. Citizens of the city-states don monstrous costumes and exchange holiday sweets with one another as part of the revelry. Large costume parties, where patrons wear either light or dark costumes (with the intention of either warding off or confusing evil) are quite popular, and those of opposite colors are encouraged to exchange gifts with one another. However they're celebrating, all across Valoran, spirits are as high as the night is long.
Does the Black Mist video erase all of this?
What do we know about autumn festivals around valoran?
Does the new Harrowing only threaten the coast and the seas?