Demon’s Souls: Telling Story Through Gameplay

The editors at Bitmob have put together an interesting but spoiler-heavy piece on how Demon's Souls utilizes gameplay to tell its story to the player.
The narrative is the story of your descent into darkness. You fall for the allure of power by killing demons to increase your soul level. By the end, you're murdering friendly humans who are unlucky enough to still be alive.

I should mention that players do have the option to lull the Old One back to sleep, and thus, save Boletaria from darkness. But such action will only immediately return the player to the Nexus for the next playthrough. At the end of the second run, players are again encouraged to join the Old One rather return him to eternal slumber -- the reward for joining is more than threefold the reward for setting the world straight.

Demon's Souls does not impart this narrative through cut-scenes or dialogue choices. There is no morality or karma system. To completely experience the game, players are encouraged to become demons themselves. By taking advantage of specific game mechanics encouraged by the game itself, player actions further the story of Demon's Souls.