Tenebrae is described as a "10+ hour adventure that is nearly a full sized release." It will be released both as DLC for Two Worlds II (for $10) and as a standalone expansion ($15) that anyone will be able to purchase and play. Either way, the game will feature a "major HD engine update" and new areas, enemies, weapons, and achievements.
Here's the official synopsis: "Two Worlds II: Call of the Tenebrae plunges the hero into a decades-old mystery and a battle against a hideous, ancient evil known as The Tenebrae. The stakes have never been higher, but our hero won't face this threat alone. Friends and foes both new and old await you, as you fight to discover the truth behind a powerful secret that will shake the Two Worlds universe to its core."
If you're looking for even more Two Worlds II content, a season pass will also be sold. This comes with Tenebrae, another DLC expansion called Shattered Embrace (slated for a release between October and December), two multiplayer map packs, and other digital content. A price for it was not shared. Call of the Tenebrae is due out on May 25 for PC, Mac, Linux, and SteamOS.