ShiroKuro Could you share what score you used?
Jane has posted the correct score -- the MuseScore link appears to be the Prelude alone, with some weird editorial choices (moving some notes from one staff to another, rewriting some notes to their enharmonic but not musical equivalent, etc.).
ShiroKuro Just how phenomenally difficult is it?
I would say it's more "tricky" than straightforwardly difficult:
- While the tempo is not particularly fast, there are some rather large LH jumps that you kind of have to take fast so that you have enough time to land softly
- It gets rather dense at points too, and there's a lot of passing the melody between the hands (common in organ music)
- Franck of course had ridiculously large hands, but us mere mortals have to decide how best to roll chords to not interrupt the line
Still, I find it an extremely satisfying piece to play or listen to, and a comfortable way to settle nerves going into a performance (a few weeks ago, due to jet lag I woke up less than an hour before I was supposed to play a half-recital 30 minutes away and rushed there in such a hurry that I actually forgot my glasses! But I switched my program order to start with the Franck and during the piece was able to settle down enough that the rest of the recital went off without a problem).