Natural Selection 2, a result of a scam that saw developer Unknown Worlds Entertainment out a total of $30,000. These 1,341 keys were purchased through the developer using stolen credit cards. The real credit card owners made chargeback claims on the fraudulent charges, and as a result the developer never received its money and was instead hit with the $30,000 fee to recoup the stolen funds.
It seems someone "purchased" keys through Unknown Worlds, sold them to third parties at a discount and then vanished once original credit card holders disputed the charges to Unknown Worlds. As a result, Unknown Worlds has temporarily closed its store, making Steam the only eligible place to purchase Natural Selection 2 at the moment.
As of today, anyone who purchased those illegitimate keys will find their game deactivated. Unknown Worlds suggests contacting the site you purchased the key from for a full refund – good luck with that.