Dark Souls 2 is a sequel to Dark Souls, which is a semi-sequel to Demon Souls. All of them are "trial by death", so to speak, and you can pick up Dark Souls on Steam today if you want a taste of it-- though I'd recommend just watching some youtube videos of the game, as the PC version requires third party tweaks to actually work (and uses the terrible Games for Windows Live DRM scheme). It is by far my favorite game of this 'generation', and it is a true challenge.
It is not an MMO, however. The world is both offline and online at the same time. You are the hero of your world, but other souls drift in and out of your world, whether to obstruct your passage (ie, they invite themselves), or to help you (you request their assistance).