Posting this on behalf of Tripwire-Stone. Both are excellent games in my opinion, KF2 for zombie co-op action and RS2 for fans of lite-sim military FPS. Both are getting updates this week and are on sale. Mind, this isn't a download and keep forever sort of deal, your access will expire at the end of the sale.
https://www.pcgamer.com/killing-floor-2-and-rising-storm-2-vietnam-are-free-to-try-this-week/
https://www.pcgamer.com/killing-floor-2-and-rising-storm-2-vietnam-are-free-to-try-this-week/