The Harrowing is basically an institution in League of Legends. It's a yearly fright-fest that ties in with the game's lore, focusing on the Black Mist and, well, death. Because it's Halloween.
The big draw in The Harrowing tends to be the special Champion skins on offer, plus various ranks of gear, and plenty of other additions to get you in the Halloween spirit.
So far, there's absolutely nothing about this year's Harrowing, but since we're well into October now, that'll likely change soon.