For those who are frustrated with the Slayer skins...
I too share a similar frustration, but let's be fair and take a look at past Harrowing skins: 2010 Lollipoppy Pumpkinhead Fiddlesticks Nosferatu Vladimir Mundo Mundo Kitty Cat Katarina Zombie Ryze
2011 Bewitching Nidalee FrankenTibbers Annie Definitely Not Blitzcrank Haunting Nocturne
2012 Zombie Brand Headmistress Fiora Pirate Ryze
2012 Shadow Isles revamp Harrowing event Haunted Maokai Headless Hecarim Underworld Twisted Fate
2013 Officer Vi Haunted Zyra
2014 Ravenborn LeBlanc Underworld Wukong
2012 Harrowing event was a huge success, and really set the standard for the Harrowing event and its skins. However, notice how every year has a champion(s) dress up in costume, with at least 1 "Undead/Spooky" skin.
2015 is no different. We get Zombie Nunu for our "Undead/Spooky" skin (comparable to 2012's Zombie Brand, not counting the Shadow Isle specific skins), and our costumed skins are Slayer Jinx and Slayer Pantheon.
Riot has been doing the same thing for the past 5 years, and this year is no different. Stop comparing 2015 to 2012's awesome event, they're not supposed to revamp the Shadow Isles every year.
As for the event, not sure what they're going to do, we're supposed to get an announcement. Keep in mind, last year, the Harrowing event of the Black Mist had some ties with Kalista, who was released just after the event (November 20 2014). The Harrowing event this year may have some ties with Kindred, the avatar of death. Who knows.
Yeah, I'm kinda disappointed that there won't be Shadow Isle skins, but that's only because I love the lore of the Shadow Isles, and it's unfair to project my expectations from a previous year's event onto the Skins team for this year's event.
FTW :-D