I do believe that he said he doesn't want just any skull, but some special kind of skill (I don't remember if the exact words he used were special, but it was something implying he didn't want ordinary skull). So I assume the writers expected you to figure out that skulls with a name attached to it is considered special and thus will satisfy Lothar.
From what I remember there are like 4 or 5 different skulls you could use to complete the quest.
The answer was on [url='http://www.gamebanshee.com/planescapetorment/walkthrough/bonesofthenight.php']our glorious walkthrough[/url].
You can provide Hargrimm's, Soego's, Stale Mary's, or the Silent King's skull from the Dead Nations or the wererat Mantuok's skull from the level below.
We live only to discover beauty. All else is a form of waiting. -- Khalil Gibran