There is a known bug in the script. See https://forums.daybreakgames.com/eq2/index.php?threads/end-of-immortality.588108/#post-6536174
In short, Decanus needs to turn into lion form before being killed in order to trigger the final boss spawn. Killing Decanus too quickly breaks the script.
Apparently this has been fixed:
I was glad to have found the cause of the issue and been able to resolve it!
Now go forth and show those Kromzek no mercy!