Foundation series
Ian Gillan introduces "The Mule" as ".. it's all about Lucifer and some of his friends, most of whom are sitting around here somewhere tonight." (From the "Live in Concert 1972/73" DVD), however on his website he also states "Yes, the Mule was inspired by Asimov.". The reference is to the character of The Mule, in the Foundation series by Isaac Asimov, specifically in Foundation and Empire.