Amazon is currently offering The Darkness II for $4.99. The last time it was available for this price (five days ago), it sold out within hours.
Once you purchase it, Amazon will provide you with a key that you can activate it on Steam with...you do not have to download the Amazon installer. Note that Amazon lists this as a "PC Download", but it's also for Mac because it's a SteamPlay title.
The Darkness II is the sequel to the critically acclaimed 2007 release The Darkness. Inspired by the popular comic book series produced by Top Cow Productions, Inc., The Darkness II is an intense first person shooter that delivers a twisted and gripping narrative of tragedy, modern crime drama, and supernatural horror. Players will be taken down the brutal and personal path of Jackie Estacado, head of a New York crime family and wielder of an ancient and ruthless force of chaos and destruction known as the Darkness.