Manes has some demos out in a compilation, Svarte Skoger. If you like the Xathur demos, this should do it for you. It's more difficult than Xasthur though.
Darkspace has a demo available for free download from their official site. Difficult as well, if you're not familiar with the band.
And of course I'm going to recommend Leviathan's demos. If you want official releases, get Verrater and Howl Mockery At The Cross. Otherwise download away at the unattainable 15 or so demos. I can vouch for Shadow Of No Light being really, really good.