As it so turns out, Alucard and Dracula plotted together to draw Satan out of hiding in order to kill both him and Death, Dracula's 2 greatest enemies. Of course, they succeeded, but it was all thanks to Alucard's efforts that the Prince of Darkness reigns supreme at the end of the game.
Revelations will chronicle what happened in between the time Alucard used the Crissaegrim to put Dracula into stasis and when Dracula awoke at the start of Lords of Shadow 2. The trailer footage shows Alucard placing the Void Sword upon the pedestal where Dracula eventually finds it during the course of the main game; the same occurs with the Chaos Claws power.
So that's how they got there! You mean it wasn't random how conveniently placed they were amongst the castle ruins?!
In short, the DLC appears to cover the time that Alucard used to lay the groundwork for Dracula's return and ascent to power.
Alucard is no slouch himself. Before he prepares the Void Sword for his father, he apparently will use it to fight enemies as well. In one hand, the almighty Crissaegrim. In his other, the venerable Void Sword.
With this DLC being set up as a prequel, it's hard to imagine it will have any shocking revelations in store. The trailer ends with Alucard facing off against Zobek's bodyguard - and we all know how that will turn out - but perhaps the DLC will have some hidden mysteries to reveal come the 25th when it's made available for Steam, PSN, and Xbox Live.