As Into The Past is a free expansion via patch (V 1.3.0.0), it should be available to everyone who owns The Demon Rush: Legends Corrupt.
Into The Past adds 3 new gameplay modes -- Side Stories (short stories where the player plays as protagonists, antagonists and side characters), RopeRange (aerial shooting range where the player must swing around the arena on a rope) and Prison Break (a party of 3 must break out of a high security prison or face execution in a week).
A preview video also accompanies the release of Into The Past, though the way work had to go on this expansion prevented the preview video from being able to be completed earlier. The preview video for the Into The Past expansion can be found on YouTube here.
Work on the 4th expansion "Warbeasts" will begin immediately, and will aim to bring a new character, a new Strategy mode (Counterintelligence), a new summoning mechanic, more Side Stories, and a new story arc to Story Mode.