You will need to buy a new starter kit that will include the new game to play, 3 Dark Skylanders characters, 3 traps, 1 two sided collectors poster, 3 sticker sheets with secret codes and 3 trading cards.
The main attraction is the five dark Skylanders: Dark Wash Buckler, Dark Blast Zone, Dark Ninja Stealth Elf, Dark Mega Ram Spyro, and Dark Slobber Tooth. And along with all the usual stickers and cards you'll get a special poster featuring all of the characters from all three games.
The Dark Element Quest Pack includes three toys as well: Knight Mare (Trap Master) figure, the Dark Spider Trap, and the Midnight Museum quest. The villain lurking in the quest is Nightshade, and the new core Dark Element Skylanders sold separately is the dragon Blackout, whose release date I am also trying to confirm.
By the way, the article in the link seems to imply that there will be more Dark Skylanders than just the five included in the Dark Edition Starter Pack. I smell a sub-series akin to the Legendary Skylanders in the air...