Most of the other miniatures and historiated initials in this book of saints are identified on the British Library’s captions with names and scenes of martyrdom. This one is labelled “Historiated initial 'I'(n) at the beginning of the passion of Eustace, with a seated monk eating pies being held in a large bowl by a devil”. The closest I can draw to explaining the connection to Eustace himself is that his emblem is an oven and that he is the patron saint of hunters, but the analogy of the devil giving pies to a monk does not follow through coherently. There is a more plausible explanation.
It could be that the initial is not connected with the story but with its moral: this story begins with the sentence I roughly translate as “In the days of Emperor Trajan [and] the devil was taking power by deception, there was a certain master of soldiers by the name of Placidus”. Placidus was Eustace’s pre-Christian name. I can’t even find the word “demonum” or any form of it in a Latin dictionary, so I am making the assumption of its meaning (a dangerous thing to do, but unfortunately I have no good medieval Latin dictionary at my disposal).
Since the book was owned first by a Benedictine abbey, it could be that for the edification of the monks, this picture was added that they might remember it when they came upon the moral of the story (as told in the Golden Legend):
“And on the morn Eustace went to hunt as he did tofore, and when he came nigh to the place he departed his knights as for to find venison. And anon he saw in the place the form of the first vision, and anon he fell to the ground tofore the figure, and said: Lord, I pray thee to show to me that which thou hast promised to me thy servant, to whom our Lord said: Eustace, thou that art blessed, which hast taken the washing of grace, for now thou hast surmounted the devil, which had deceived thee, and trodden him under foot, now thy faith shall appear. The devil now, because thou hast forsaken him, is armed cruelly against thee, and it behoveth thee to suffer many things and pains. For to have the crown of victory thou must suffer much, because to humble thee from the high vanity of the world, and shalt afterward be enhanced in spiritual riches, thou therefore fail not, ne look not unto thy first glory. For thee behoveth that by temptations thou be another Job, and when thou shalt so be humbled, I shall come to thee, and shall restore thee unto thy first joy. Say to me now whether thou wilt now suffer and take temptations, or in the end of thy life? And Eustace said to him: Lord, if it so behoveth. command that temptation to come now, but I beseech thee to grant to me the virtue of patience. To whom our Lord said: Be thou constant, for my grace shall keep your souls.”
Applying this moral to the life of a monk is fairly simple; their vows were of poverty, obedience, chastity and stability. Of course, since they had chosen to follow Christ the devil was “armed cruelly against [them]” by temptation to break these vows by a number of ways. Gluttony was also one of the Seven Deadly Sins, so the devil with a pie is a fairly simple admonition to endure through religious fasts to gain “spiritual riches”. I would note that I’m not sure if Eustace, even with his ovens and temptations, was ever exhorted for endurance during a fast--he was also the helper to avoid family discord, but I can’t find evidence for a direct relation to a fast.
Unfortunately, I’m not privy to the entire MS or transcription, or I’d compare the vocabulary between the “deception” in the first sentence and the passage in which God speaks to Eustace about “the devil which had deceived [him]”, and the rest of the moral tale in this particular MS, since it isn’t corresponding to the Golden Legend version and might have another passage entirely!
Image from Arundel MS 91, f.190 provided courtesy of the British Library.