Considering how much I hated Demon's Souls, no chance I am ever going to play Dark Souls. There is challenging and then there is just giving enemy stupid stats, I found Demon's Souls to be the latter rather then the former it's difficulty was not because it's a challenging game, it's difficulty is the fact that the player is a sheet of paper that gets torn to shreds to the smallest gust of wind. Also if Dark Souls has the same broken multi-player as Demon's Souls had then my words are hell, no.
Skyrim can be set to master difficulty but that's just going the same way as dark souls, making the difficulty only a thing of boosting enemies stats so much that the player becomes a sheet of paper rather then actually difficult. Action RPGs should be more then just learning an enemies move set and then abusing their AI until they fall over dead. Now I get there are people that like these grind quest games like Dark/Demon's Souls but to me they're hollow games with little to actually offer over going to the local S&M club because your idea of difficulty is about being beaten like a ragdoll more then it is actually about being "challenging".
To this respect I'd say fallout 3 was a more challenging game then either the of the Souls games and more challenging then skyrim. You aren't a paper sheet but if you just run into situations head first you'll be handed your backside on a platter by the local friendly neighborhood super mutant.
The only thing I real liked at all about Demon's Souls was the boss fights, they were more epic then most games but I've played a game before with even better boss fights, Phantasy Star Online... fact is boss fights in games these days are a joke and even the best boss fights often don't match up to those of 10 years ago

.