Comment the hardest game you have ever played (but not necessarily beaten).
[spoiler]Dark Souls[/spoiler]
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I don't know if you mean general difficulty but since you said "beaten" I'm assuming you're talking about the most difficult campaign. If it's overall difficulty? It would have to be EVE online. Although I wouldn't even call that a game (because its that hard). If it would have to be a campaign in traditional terms? It would probably have to be X-com: Enemy Unknown on impossible.