Why is Darksiders marked as FREE here?

This is usually because the store page has no price and may be due to giveaways linking to the original which has been purchase disabled and as such also has no price.

I had no idea about it, it's in my library. hum. Is it maybe because what eggshen said.

Achievements on the new version sems to be buggy, and it seems there are the same. Nothing new. This new remaster thing is quite sad

It is because it is no longer for sale, so like zero for unpurchasable.

It's a Steam API issue. I don't think anyone can do anything about it though

Not free cz of given away, it just giving 0p.
Can't buy Darksider anymore (or at least for now).
Only "Buy Darksiders Warmastered Edition" available

The original game's sub has been removed from sale, that's most likely the reason, forcing the game's price to be returned as 0.
Not sure what SG staff will do with this one though, since the original version has been removed from the giveaway list (automatically, due to the price issue), while you can still be granted the original version if you activate a copy of the new version (meaning selecting the original version or the new version is essentially the same except for the old giveaways don't get CV).

Damn, and I just got back to Level 6 after previous drops to 0p (Bioshock, Titan Quest).

Time to make more gibs, I guess