Darksiders contained an insert (see above) that quite explicitly asked buyers to not to throw it away -- that's what "do not discard" means. Today it was revealed that the code will net the bearer a free copy of Red Faction Guerrilla for PS3 or Xbox 360. The code will also grant them "a digital Hellbook disc featuring Darksiders art, videos, and other awesome stuff." Yeah, that's great but let's get back to that whole "free game" thing.
The offer is only available in the US and ends March 7, 2010. We've got a call in to THQ about whether recent Darksiders printings still have the card. Of course there's a catch: there's a $5 shipping charge; however, we can guarantee (because it was all over our "Best of the Rest" for 2009) that $5 is an incredibly small price to pay for hours of joyous Martian destruction.
[Thanks Salvador and Stephen McB]