Still an extender, not familiar with their Interlude version, but I have seen their Epilogue version and had some obvious flaws that Java servers do not have.
Extenders do have a good base to begin with, but what is added is no better that what L2-Scripts or any other Java project do.
In general I do not know what to believe, unless tested on real retail servers or seen in some video that seems to be from a retail server.
At the end it is up to you to decide, just do some research on old lineage 2 forums, wayback machine,
...or even the retail files you have.
Back to this skill...
From client description.
The spirit of an ancient assassin temporarily possesses the user.
Nothing to point to a proper direction.