Piano Forum

Well, from Brahms, either the Op. 118 no. 6, or either of the Rhapsodies Op. 79.

If you want pretty dark, you could surely do a better job than i am with Scriabin's Op. 74 préludes right now.  I'm not going to dare mention any of Scriabin's later sonatas, mainly because there's absolutely no way I can recommend something I am very far being able to play, both technically and musically.

You have a lot of options in Bach, which you should probably pick and choose for yourself.  Or you have the Op. 26 sonata of Beethoven, or even some of his bagatelles from Op. 126.

Prokofiev's Seventh Piano Sonata.  I wouldn't say dark, but very heavy.  I'm not sure if the distinction is meaningful.

And since you mention Schubert, many of the pieces from the Winterreise are rather somber.  Of course, they're songs, but you could play the accompinaments pretty easily, and, although it's been a while since I looked at the parts, I think you could cover the vocal parts in some if not many cases.

Berg's Op. 1 piano sonata is not overly difficult for technique, and it's rather dark, but it's quite a handful to actually play it beyond just sight-reading it here and there.  I'm fairly sure someone here analyzed this piece for at least a chapter of his doctoral disseration, and I'm also fairly sure that many people here have played this work to a better level than me just plinking around with it over the years.