The first piece of DLC for L4D2 arrived in April 2010 and was followed by a prequel, titled The Sacrifice, later that year. A community-create content pack called Cold Stream came in mid-2012 but since then, the trail has been cold… until now.
https://www.techspot.com/news/86540-valve-drops-trailer-community-created-left-4-dead.html?utm_source=dlvr.it&utm_medium=blogger
https://www.techspot.com/news/86540-valve-drops-trailer-community-created-left-4-dead.html?utm_source=dlvr.it&utm_medium=blogger