Insurgency: Sandstorm is the sequel to the popular 2014 FPS, Insurgency, which sold over five million copies. The original game is built on the Source engine while the sequel is powered by Unreal 4. Both games look great and if you already own the first one, you can get a 10% off loyalty discount on the sequel.
Source: N4G PC Insurgency: Sandstorm review – ChristCenteredGamer