As I replied to your support email earlier:
Did you check the castle area with Alt held down, or Toggle World Tooltips on in the controller UI?
If there are a limited number of places where you may have been sorting your inventory after that fight, you could look there, as well.
Did you double check character inventories, the homestead chests, etc? Also check if a character may have the staff equipped.
If you can not find it, please reply to the email with your latest save. Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\PlayerProfiles\<ProfileName>\Savegames_patch' folder.
In Windows Explorer you can zip a file or folder by right clicking on it (or them) and selecting 'Send To | Compressed (zipped) Folder' (and similarly in OSX/Finder).