As Tenebris is a free expansion delivered via patch (V. 1.2.0.0), it should now be available to everyone who currently owns The Demon Rush: Legends Corrupt. Most of these additions are also listed in the "Tenebris" small update/patch notes announcement.
It's a pretty hefty expansion, too -- it brings the new playable character Caesara, the new Naval Conquest gameplay mode, a new story arc (that contains a new world, new main quest, new side quests, new superdungeon, new endgame Challenges, new weapons/passives), the Wild Nova ability for Barklord, and gameplay additions for Endless Dungeon, Conquest and Heist.
Work on the next expansion, Into The Past, will begin immediately. Into The Past intends to add 3 gameplay modes -- Side Stories, RopeRange and Prison Break. As it is intended to be a shorter expansion, it should hopefully have a faster development time than Tenebris did.