There neuer was a better bargaine driu'ne
Crocell, Duke of Hell and ruler of the forty-eight legions, was distinctly baffled. He had passed through anger at being summoned and confined, bemusement at the notion that his summoner was genuinely a small child, glee at the idea of corrupting an innocent witch child, amusement at her notion of 'nice presents,' concern over her awareness that bargains with demons often ended badly for the mortal, interest at the prospect of a fun negotiation, and now he was back to bafflement at how much the mortal world seemed to have changed, not just in terms of the practical things non-magical humans could do, but also in the ways that the conceptual frameworks under which their society operated had shifted.
So far, they had only negotiated an agreement for this one visit. She would let him out of the circle, and he would be able to roam freely within the dwelling; he promised not to hurt her, her parents or her nanny on this one visit. They would converse, and both were committed to tell the truth insofar as they knew it, to refuse to answer if questions were too intrusive but not to lie. Among the rather paltry offerings, he had at first disdained the still picture of a deciduous forest, assuming it to be a simple recordari charm, and a paltry one with an image that didn't even move. But when she explained what a magazine was, and newspapers, and how they differed from books, he was fascinated, and the child was only too happy to run downstairs and fetch him some newspapers, and make him a gift of the ones that had been thrown away by her parents because they were several days old and therefore no longer valuable. She was apologetic that her parents might miss the current ones that, apparently, had a place on the 'coffee table' in the 'living room' until such time as they, too, were discarded. These were clearly the descendants of the broadsheets he had glimpsed when summoned in the sixteenth century, but instead of a single large page or a tiny booklet, these were large and voluminous, and instead of having a readership of hundreds, they had millions.
But his bemusement over the dramatic increase in the population, and even over the apparent fact that the current mortal society had no official magic-users, and seemed to be of the belief that magic was not real; even that was nothing to his bemusement as to the state of affairs with regard to religion and ethics. Of course he was aware of the Protestant Reformation; every demon was, even if they had not been summoned recently enough to have personal experience with differing Christian sects. It had been such a major upheaval throughout Europe. But it had been his understanding that the humans had instituted a rule whereby every country was either a Protestant or Catholic country, and every subject was obliged to follow the faith of the country they lived in: cuius regio, eius religio. And yet apparently that was not the case for twentieth-century England. They still had a Church of England, and the education his summoner received was somehow connected to it (worrying), but she and her family were 'all atheists,' and this was in no way secret, shameful, or unusual. No adult citizen was compelled to go to church, and while many were members of the Church of England, there were also Catholic churches, different Protestant churches, and places of worship for people with other religions, and individuals could go to any institution they liked, or choose not to go to any, and nobody thought anything of it. Well, the state didn't, and while Hermione was vaguely aware that some people thought everybody should believe and worship as they did, those people were thought to be somewhat odd, 'fanatics,' by society at large. Excessive religious zeal was seen almost as a mental aberration.
And with religious faith being something one chose, rather than just something one grew up in, unquestioning, these modern humans had concepts of 'ethics', 'morals', and 'being a good person' that were wholly separate from religious piety, codes that were not dictated by a deity but worked out from first principles, though they often ended up bearing a strong resemblance to those moral codes that were derived from religion. Free will in particular - that area where Crocell and his ilk had diverged so strongly from the demands of the King of Kings - was something deeply valued by Hermione and her parents. She had very strong views on slavery, on bodily and mental autonomy, and on self-determination; this might well, Crocell felt, be enough to offset her inconvenient and equally strong belief in fairness, in justice, and in kindness over cruelty. She was, at least, aware that these values could clash, and that when they did, 'you just made the best decision you could.' And she was young. Clever, yes, knowledgeable, yes, but still naive, still influenceable. And she wanted a friend, a mentor. Such a person would of necessity be able to influence her.
Yes, he decided, she had the makings of a Dark Lady, and even if she never became one, she might still be an amusing diversion, and a way of learning more about the mortal world, in order to increase the influence of the Dark within it, and further its cause. She was a determinedly righteous person, and stubborn - but those qualities had their uses. And so they took their unbreakable vows. The basis of them - ten years during which he would come when summoned, and teach her various subjects - was what he had initially planned to offer. There were more caveats in than he would have liked: she would attempt sincerely to learn Dark magic spells and theory to the best of her ability and understanding, with increasing freedom, as she advanced, to choose between different sub-schools according to her tastes, preferences and ethics as well as skill levels. The field of Dark Arts was vast enough, after all, that even if she never came round to carrying out unwilling human sacrifices, or subverting the will of sentient beings, or binding them or modifying their bodies without their consent, there was still more left to learn than a single mortal lifespan had time for. And in return for that extra flexibility in her promise to learn Dark Magic, as well as vowing never to summon Crocell in a manner that imprisoned or constrained him against his will, her commitment to Crocell's side of the Eternal War was made rather firmer than simply 'never knowingly aiding an angel.' She also swore never to pledge her soul to Christ or the King of Kings, to respect and honour Crocell as her mentor, to be discreet about her contacts with demons around people who might not approve (which was not the same thing as total secrecy), and (when it was possible, practical and not too inconvenient) to help him keep himself up-to-date with the fate of his distant relations here on Earth. She was not bound to obey him; she could still disagree with him and disregard his advice, just as she might that of a parent or a teacher; but, as with a parent or teacher, she would listen seriously to what he said, and depart from his guidance when she felt she had good reasons for doing so, and argue or evade respectfully.
They didn't put it in the Vows, but they also agreed that he could have all the household's discarded newspapers as a matter of course, and he would be delighted to attend tea-parties, and to see which of his friends and colleagues might like to attend.
He had stretched the terms of the treaty more than a little, giving her as much information as he had on the Eternal War, and on the notion that witches, wizards and magical creatures were distantly descended from angels and demons. But she was bound to discretion, if not secrecy, and he was prepared to argue to King Paimon, who held his allegiance, that her promise of limited information-gathering in return, and her acceptance of his mentorship, would give them a foothold in the material world that had the potential to more than counterbalance the magnitude of the gift.
And she was clever, and earnest, and hungered for friendship and acceptance as much as knowledge. Mentoring her for ten years would be no hardship at all. He was no child, to pledge friendship and immediately feel it. But he wasn't wholly unmoved, either.