Riot has begun League of Legend's Halloween themed event The Harrowing, which introduces Mystery Skin Gifting among friends and turns your sight wards into spooky things like Tombstones and Evil Trees and other such kitsch symbols. In addition, there are two new hero skins on sale, a police offeicer version of Vi and a spooky version of Zyra, for about $8 each.
In addition to being able to purchase these new skins, Riot's going to kindly let you buy some legacy skins from previous Halloweens, so grab those if you missed them the first time. They really are quite nice.
The big new thing is mystery gifting, which allows you to send a 490 RP gift of a random skin to a mate. The skin will always be for a hero your friend owns and be worth at least 520 RP. However, the gifting is so far only planned for the Harrowing event, so get on that if you fancy it. Much like purchasing just about anything, buying a mystery gift will award you a summoner icon at the end of the event, because Riot's kind like that.