So, if you've got a copy of Darksiders handy, you may also still have that mysterious promo code.
Seems THQ is finally letting you use the darn thing -- for a free copy of Red Faction: Guerrilla and the Darksiders Digital Hellbook disc. The Hellbook, if you didn't already guess, comes with Darksiders art, videos, and other related content.
Of course, this does mean there's a small shipping fee, even if everything else is free. THQ was kind enough to provide links to both the PS3 and Xbox 360 versions, so you can skip a few steps and go directly to check out. Do remember to actually use your promo code.