The main plot crux of Susanna Clarke's novel Jonathan Strange and Mr Norrell is to be found in the chapter entitled "A gentleman with thistledown hair", in which Norrell becomes the first English magician to summon a fairy and ask for his assistance for perhaps four hundred years.
Norrell is an extremely dishonest magician, and (despite doing exactly this) consistently argues (in public and even with Jonathan Strange) against the practice of magicians contacting fairies - because of the dangers; and this warning superficially seems to be amply justified by the subsequent events.
Nonetheless, it also becomes clear that all magic ultimately comes from fairies - and that even Norrell's own magic, which he believes himself to have learned entirely from books, has been tacitly permitted, and indeed encouraged, from Faerie by the Raven King.
So, I think we must assume that it was, in principle, alright for Norrell to enlist the assistance of a fairy - if, let us say, the thing was done for the right reasons and in the right way. But that in fact there was something about the way this fairy was enlisted by Norrell which led to many bad outcomes - the actual summoning was (we can infer) done for the wrong reasons and in the wrong ways.
Firstly, Norrell enlisted the fairy primarily for his own self-aggrandizement - in order to impress the prominent politician Sir Walter Pole by raising his recently-dead fiancee, and thereby putting Sir Walter into Norrell's debt. Norrell justifies this to himself by pretending the action was for the benefit of restoring 'English magic', but in practice Norrell always interprets 'English magic' purely in terms of himself and his own benefit; and he does his best to prevent or suppress all other aspects of English magic.
So first the motive was wrong. What, then, about the specific fairy who was summoned: the 'gentleman with thistledown hair'? At first glance he seems to be the worst possible fairy that Norrell could have summoned - however it seems that Norrell had no control over this, and I think we must assume that the Raven King was behind the choice.
The thistledown fairy has, indeed, many desirable qualities, some of which he tells us (and seem to be confirmed) and others which emerge through the story. Firstly, he is a King and is perhaps the most magically powerful fairy alive; and secondly he has been the servant and friend of some of the greatest magicians England has ever known: primarily the Raven King himself, but also the Golden Age ('aureate') magicians Thomas Godbless, Ralph Stokesy and Martin Pale.
Although there is considerable evidence that this fairy has become corrupted over the past few hundred years, and is now a monster of cruelty and conceit, I think that we would be justified in assuming that if Norrell's motives had been good and if Norrell's decisions had been wise and altruistic, this fairy would have been suitable.
The crucial moment comes when the 'bargain' is being settled between Norrell and the fairy - when the fairy asks: "Should I agree to restore this young woman to life, what would be my reward?"
Norrell asks what the fairy wants, and the response is: "to be allowed to aid you in all your endeavors, to advise you upon all matters and to guide you in your studies. Oh, and you must take care to let all the world know that your greatest achievements are due to larger part to me!"
This seems not unreasonable, and would seem to be the normal way in which fairies have worked with magicians in the past - they are apparently a conceited race and love nothing more than frequent praise and honours; but have been content to be led strategically and ruled by Men, who are more diligent than they.
However, Norrell rejects this request for selfish, egotistical reasons, without consideration; this, because he wishes to be given personal credit for all magic, and does not want to share status or praise with anybody - least of all with a fairy.
This refusal provokes 'a long silence' and then the fairy declares 'this is ungrateful indeed' - and eventually suggests a deceptive 'deal' where he claims 'half' of Lady Pole's new life - which Norrell simply understands to mean her lifespan being shortened by half; but which the fairy interprets to mean absolute control over half of each and every day of Lady Pole's life.
(For half of every day for an agreed ninety-four years, during the nights, Lady Pole is therefore compelled to be the fairy's companion in fairyland; engaged in repetitive and tedious dances, rituals and ceremonies - until she comes to regard her inescapable fate as literally worse than being dead.)
What has happened is that instead of Norrell himself paying the 'price' for the fairy's cooperation, he makes Lady Pole pay the price - whether she likes it or not. This is the essence of Norrell's wicked action.
So - I think we can locate Norrell's refusal of the fairy's first offer as the critical turning point in the plot of the book, since it leads to the enchantments of Lady Pole, Stephen Black and Arabella Strange - and to the fixed hostility of the fairy towards both Norrell and Strange (leading to their own bleak personal fates as co-prisoners bound in darkness).
In sum: it was necessary to the restoration of English magic that a fairy be summoned, and it was probably right that that particular fairy be the gentleman with thistledown hair; but most of the tragedies of the story stem from Norrell's selfishly bad motivations in summoning the fairy and his selfishly bad decision with respect to making a deal with the fairy.