It's not often you see a genuine "expansion" to an existing adventure game, but that's exactly what's happening today, as Paradox Interactive has now released the download-only Penumbra: Requiem on GamersGate.
Beginning with Overture, the Penumbra series was originally planned as a trilogy, then scaled back to two games before its unanswered questions inspired a third and final instalment after all. The storyline of Requiem picks up where last year's Black Plague ended, with the game's protagonist "falling deeper into madness which forces him to fight some inner demons as well as outer ones." The new game isn't just a continuation, however, but an expansion that requires its predecessor to play.
Update: To coincide with the game's release, a gameplay trailer for Requiem has now been released, which can be streamed directly from YouTube.
For those who already own Black Plague, Penumbra: Requiem can be downloaded alone for only $9.99 USD. Alternatively, since Requiem isn't a standalone experience, a Gold Pack is also available with the two games combined.