Dead Sun was a canned Soul Reaver/Legacy of Kain follow up, the multiplayer component of which became the F2P Nosgoth. Bits of it have been seen before, now there's half an hour of gameplay.
According to a new NeoGAF post by user Mama Robotnik (who's already done some excellent research on the game) 2012's Dead Sun was ultimately cancelled because it was "not projected to meet Square Enix sales projections". That despite, as the video below suggests, the game's relatively advanced development.
The story follows a human, Asher, who possesses a vampire's body. The vampire, Gein, then becomes a sort of spirit guide to Asher who gains a similar world-shifting mechanic to the original Soul Reaver. One post, from someone claiming to have been a developer who worked on the project, provides some additional info:
"I worked on this game. Glad something has come out of it, even if that's just excitement from people who can see what we were going for. (And this was running on 360, so remember that - the creaking you can see was the engine running two entire levels at the same time, so that you could shift realms at any point, something that UE3 really isn't designed to do.)
I hope somebody leaks some videos of other parts of the game. I don't have any, annoyingly, but others who worked on it might have a video of The Temple Of The Whore Saint, which was one of the dungeons. The thing to remember is that this bit is a small chunk of what was essentially Hyrule field - an expanse designed to lead from place to place. The person playing this is really taking their time with a bit of the game that has zero missions - the actual structure was far more Zelda/Soul Reaver than this might appear."
Here's the video ("supplied by a source who worked on the game in some capacity") which is labelled "Vertical Slice Presentation". It shows The Wetlands, a hub area that would have tied all the other locations together. Also mentioned in the debug menus are The Fireship, Lydia's Shrine and an unnamed boss.