Death to spies is a hardcore stealth game set in Soviet WW2 that has developed a reputation for its difficulty.
Worth checking out if you are a fan of stealth series such as Hitman, Splinter Cell
Death to Spies: Gold Edition includes both Death to Spies and Death to Spies: Moment of Truth, giving you 2 full games. Based on real WWII operations you must venture behind enemy lines to steal classified documents, gather intelligence and eliminate officers.
This bundle is currently available on Steam for £19.99