Old school stealth 'em up Styx: Shards Of Darkness is currently free to keep on GOG
As Alec wrote in his Styx: Shards of Darkness review, what the goblin stealth 'em up has going for it is purity. It's a third-person adventure where stealth is the only option and being spotted is swiftly punished.
It's currently free from GOG.com.