Did you talk to Jonelath again after that?
There was a bug where certain opponents could be missing, which was fixed in the patch (though if the problem had already occurred in the save the patch might not correct it). If that is the case here, and you have installed the patch, email support@larian.com with a description of the problem, and maybe a link to this topic, and with your save they should be able to restore the ultimate creature.
See Lar's post
here on how to send them an Xbox save.
With the PC version, in XP the saves are in the folder
C:\Documents and Settings\ %username% \Local Settings\Application Data\Divinity 2\Savegames
or in Vista / Win 7
C:\Users\ %username% \AppData\Local\Divinity 2\Savegames
You may need to set Windows Explorer to show hidden files and folders (
XP, Vista,
Win 7).