I feel like it's just promotional material for the Harrowing. The fog on the splash arts for Cassiopeia, Hecarim, Lissandra, LeBlanc, Xerath, Twisted Fate, and Zed seem to signal a fading into darkness, while the time of day changes for Braum's and Yasuo's splash arts, and a large amount of darker tinting in many splash arts including Volibear's, Heimerdinger's, Ashe's, and Brand's splash arts, among others that I may not have found, all lead to suggest that this is elaborate promotional material spread throughout the official League of Legends web site. Riot has also suggested that they wanted to make this Harrowing special this year, with two new skins ready to be released, another skin being tested on the PBE, and even a Hexakill Twisted Treeline featured mode (tested on PBE) all point to promotional material for Harrowing.
I doubt that the shadows would signal a new champion release for several reasons. Throughout the year, champion releases have been very spread out until some of the most recent releases, with the release of Gnar, the subsequent and quite possibly rushed release of Azir to come out with the Shurima event, and the Sion relaunch, which can be termed a new release. With all of the recent champion releases and Riot's tendency to spread out champion releases more, it makes very little sense to release another champion release in such a short time span.
Also, all of the champion previews show some level of the character's thematics, whether it was the monstrosity of Mega Gnar, the Trials of the Poro video, or the mysterious Vel'koz video clip integrated into the videos of many popular league of legends Youtubers. The obvious thematics of this new champ seem to mark the use of shadows, but we already have heroes that use shadows and darkness in Zed and Nocturne, while the symbolism of the shadows, which could mean deception, has been used by several heroes, including LeBlanc and Cassiopeia. Riot's recent champion thematic design philosophy is to do a thematic good once, and as good as possible, and then never touch on that mechanic again. If this is a preview, then it will not be for a champion.