This innovative title from 1994 is a first-person sci-fi/horror game set aboard the Citadel space station, where the player has to fight mutants and robots created by the evil, yet brilliant, artificial intelligence called SHODAN. The game could look like any other first person shooter at first sight, but its detailed environments, engrossing story-line and one of the creepiest villains ever makes it a true classic which inspired games like Deus Ex and Half-Life. Sadly, it was outsold by mainstream games a la Doom.
System Shock 2, the sequel to System Shock, was developed by Irrational Games and Looking Glass and released in 1999.
This game takes the tenets of its predecessor, namely to combine a sci-fi/horror action game with role-playing elements, one step further by relying on the modified Thief engine for visuals and atmosphere. The result is one of the scariest and most engrossing first-person games ever made - playing it in a darkened room and with a headset on is not for the faint of heart.
Similar to the earlier game, the story takes place in an artificial construct (a huge spacecraft this time) that has been taken over by an alien intelligence, basically turning all the surroundings against the player and providing a nicely paranoid invasion setting (think Half-Life without the optimism). The character can be customized towards various professions, like gun-slinging Marine, engineer or psi-corps semi-wizard. Gameplay requires quite a lot more variabilty and alternative problem solving than most first-person shooters, mainly because the shooting part has been roughened up - most guns require special skills, and skill points are hard to come by; also, guns degrade through use and have to be repaired. In a typical game, the player will only be able to use about half of the weapons (depending on choice of specialization) - this provides a good amount of replayability.
A free fan-made add-on pack is available on the net that improves some of the textures and polygon counts that had been kept simple to ensure high performance speed.