There is a mystery about the Montfaucon portrait of Gilles de Rais, which should properly be called the Bonnier portrait, from Gilles le Bonnier, who commissioned it; Dom Bernard de Montfaucon merely published a version of it in his Les Monumens de la Monarchie Françoise in the early 18th century.
The portrait was almost certainly made after Gilles' death, but Bonnier had been familiar with him, at least by sight. He was otherwise known as Berry-King-at-Arms, or First Herald of France, under Charles VII and his father. The manuscript of which the portrait forms a part was presented to the former, who also knew Gilles.
Montfaucon clearly labels the portrait "Gilles de Laval", adding that this is the Marshal who was executed at Nantes; the text apparently was taken from Berry. So it seems from its provenance that this is the only authentic portrait of Gilles de Rais in existence. The problem is that, instead of his familiar arms - d'or à la croix de sable - we see the arms of Montmorency: or on a cross gules, cantoned with sixteen alerions azure. The addition of five scallop shells specifies the Montmorency-Laval family, but not Gilles' branch of it.
Since Bonnier knew Gilles, and was an expert in heraldry, it seems unlikely that he would have allowed such a grave error to slip by, so most scholars have assumed that the figure in the portrait is actually Guy de Laval, his cousin. However, this also involves a serious error, since the original text was very specific - it was not simply a question of a scribe accidentally writing "Gilles" instead of "Guy", since the latter was not a Marshal of France and nor was he executed.
E A Vizetelly, the biographer most vexed by this issue, favours the theory that a scribe erred, but adds: If our surmise be inaccurate, and the figure be really that of Gilles de Rais, we can only assume that he, on entering the service of France, discarded the arms of his Breton barony to bear those of his Montmorency ancestors, regardless of any agreement into which his father had entered.
Obviously he thinks this theory to be quite outlandish, but is it? Gilles would have been a Laval if his father had not covenanted with Jeanne La Sage to take the name and arms of Rais. We know that he identified as French, espousing the unpopular and apparently lost cause of the Dauphin. We also know that Brittany allied with England as often as with France. Perhaps Gilles found it more appropriate to fight under French arms rather than Breton ones.
There may be a link to the sale of Blaison in 1429. This was the first property Gilles sold, probably to finance his troops, and it seems significant that it was his father's patrimonial estate. This looks like a meaningful gesture that indicates some animus between father and son. Possibly Gilles resented having to bear the arms and name of a Breton barony when his loyalties lay with France, or possibly there had been some other dispute. In either case, since he disposed of Blaison without a qualm, he would certainly not have hesitated to dump the Rais arms and break his father's covenant.
Would Gilles really have turned his back on the black cross of Rais and the agreement that had made him the heir to massive estates in the Pays de Rais? It would have been a shocking and controversial move, but it would not be the first or last on his part. We should not rule out the theory that the Montfaucon portrait represents him bearing arms that had been renounced by his father before he was born.
Gilles de Rais was a man who made a career of the improbable.