Exactly how Dark are these Souls?

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I played a little bit of Dark Souls at a freinds house a few days back, it really intrigued me.

Could someone maybe fill me in on what the games all about? Does it have a story line? Are you just supposed to forge your own path through the bright but still depressing world that I saw? From what I did pick up though, it's brutally difficult, I loved that part, it felt like it was kicking my ass but at the same time It was absolutely my fault I was losing.