The Killing Floor franchise is rather popular.
Tripwire Interactive announced its co-op first person series Killing Floor, has sold nearly 10 million units worldwide.
Since the first game released in 2009, Killing Floor titles have earned over $100 million in revenue, and the franchise has been played by over 15 million unique users.
Content such as numerous free updates and mod support are just two components to the franchise's success, according to Tripwire (thanks, Blue).
Killing Floor 2, as an example, has seen quite a few season updates along with over 49 new maps and 139 weapons. More are on the way, too.
To celebrate the milestone, and the series' 10th Anniversary, the publisher is hosting a sale on Steam through May 20.
The sale includes discounts on the firm's entire Steam catalog including last week’s Spring Content Update for Rising Storm 2: Vietnam.