Here are the patch notes, via the official website:
Additions and Changes
- Added “Lunatic” difficulty.
- Adjusted the retry respawn point in the “Prisoner” chapter dungeon.
- Added the ability to skip the Prisoner Dungeon on second playthrough.
- Adjusted the effect of the “Gale Shoes” to be easier to understand.
- Extended the effect time of “Shadow Dive” and added an HP absorption effect.
- Adjusted player character performance.
- Increased the angle at which you can look up when controlling the camera.
- Improved frame rate.
- Improved operational stability.
- Fixed a bug that caused the game to crash or freeze in certain situations.
- Corrected typos and omissions.
- Various other bug fixes.
Ys IX: Monstrum Nox is availble now for PlayStation 4 in Japan. A western release has yet to be announced.