Discussing in TalkTull, Jan Voorbij, amongst others, questioned the then-accepted assumption that the word 'perdé' was from the mediæval French "par Dieu" (by God):
"If you check the lyrics of the original APP cover you can see that it is not Magus Perdé. There is a horizontal line over the last 'e' of perde, which in the Middle Ages meant that the word was abbreviated, a common practice in Western Europe. This was done to save space for illustrations and illuminated capitals. The educated reader knew perfectly well what was meant."
Jan's brother, a doctor of mediæval Dutch, German and Latin concluded that it could only be a contraction of the Latin "perditio".
Both explanations are equally credible. Magus Perdé could be a magician/wise man invested with power "by God", or bearing godlike power. Alternatively, the Magus may simply be "perditio" (damned); after all, Ronnie encounters him in Hell.
However, as Andy Jackson pointed out, for the '-' theory to be correct, Ian would need to have seen a copy of an original Latin manuscript, then picked out this word at random - quite a feat considering line-by-line annotations aren't possible, and full calligraphic texts are not the easiest things from which to pick out individual words, especially if you don't know Latin. It stretches credibility to imagine Ian poring over a reproduction Latin manuscript at 2 am in a hotel room somewhere, following a word-by-word translation from one page to the nextů.
It's far more likely that the '-' on the original album sleeve was merely a limitation of the typeface. In the accompanying Linwell Theatre programme, it was clearly 'é' (Magus Perdé's drawing-room at midnight). Andy suggested that the typeface used for the sleeve lyrics simply didn't include a French acute accent, so they improvised an ordinary '-' placed over the 'e'. The Theatre booklet used a different typeface, which did include an acute accent, so was properly used. In the piece itself, Ian distinctly pronounces the word with an acute-accent 'e', as in standard French, so one would have to doubt the idea that the horizontal-bar 'e' was intentional , representing a mediæval calligraphic convention. Elegant as the '-' theory seems, it can only be a red herring.
Yet one can readily cut through this entire discussion by questioning the idea that it's mediæval French at all: why go for the complicated explanation when there's an obvious one? 'Perdé' is modern French for 'lost' (3rd person singular). The idea that he's 'damned' is uncontentious.
So who is The Magus, and why did Ian so name him?
I'm afraid it's still a mystery. It has been suggested that Magus Perdé was the author of a medieval Passion play. However, there's no evidence to support that idea. Certainly in Britain between 1375 and 1575, Passion plays were communal efforts, an oral tradition rather than the works of individual authors. The same situation may be assumed to have applied in France; extensive web research has certainly failed to identify such a person.
Some commentators have drawn a parallel between the word 'Magus' and the Magi of the Christian Nativity. That is the most likely context in which a listener, particularly from a Christian background, may have encountered the word, but it's certainly not the only context, and there's no real reason to link the 'three wise men' to the character in The Play; nor does it remotely fit the narrative.
Asked by Carsten Bergmann in 1976, Ian's response to the question "Who is Magus Perdé?" was "It's a mixture of Latin and French".