Brandon Sanderson has three laws of magic. The first is: An author’s ability to solve conflict with magic is directly proportional to how well the reader understands said magic. This means a few things, which you can read more about here, but I’m focusing on two aspects: magic has rules and magic is used to create problems, not solve them. This way we don’t end up in a Marvel-esque deus ex machina where suddenly our hero is extremely overpowered and disproportional to what we’ve seen previously. To give an example, and a spoiler for Fantastic Beasts and Where to Find Them, at the end of the movie, our heroes memory-wipe the entirety of New York with a small magic vial. It was frustrating because it felt lazy, boring, and overpowered and it was never established that you could do that to that scale. But most importantly, it was no more and no less 
than an easy fix to a difficult problem. There’s no story to it, no examination of the effect wizards have on everyday people or what it means to have to keep your existence so secret. An example like this shows us that rules are very important. But let’s break them anyway. At the midpoint of The Last Unicorn, Peter Beagle breaks his magic system and does so in order to solve a problem.
From the book jacket:
The unicorn had lived since before memory in a forest where death could touch nothing. But outside her wondrous realm, dark whispers and rumors carried a message she could not ignore: “Unicorns are gone from the world.” Aided by Schmendrick, a bumbling magician, and Molly, an indomitable spinster, she set out to learn the truth.
At the midpoint of The Last Unicorn, the unicorn is being pursued by a surreal, fairy-tale caricature, the Red Bull. The wizard Schmendrick has very little control over his magic, and although he’s done one great feat of magic previously, there’s little rhyme or reason to when he’s able to tap into it. Yet now, desperately trying to help, he saves the last unicorn from the Red Bull by turning her into a human.
It is, in many ways, a deus ex machina. It’s a grand act of magic that saves the day, and not by any skill or rules, but because the plot demands it. His magic should be boring. And yet it’s wondrous and mysterious and fascinating.
The characters in The Last Unicorn spend a lot of time escaping bad predicaments, but they escape because people are too caught up in fake fantasies to see the real fantasy in front of them. The fantasies aren’t fun or even satisfying. A sense of foreboding hangs over everything and more and more it seems like escape isn’t worth it, like the unicorn never should’ve left her cozy forest in the first place.
Successes in The Last Unicorn are not cause for celebration. They are almost always sad affairs. The potential for success is always mired in the fear of failure. The joy of success is always tempered by the knowledge that you won’t be successful forever; you’ll have to keep going, you’ll get old, you’ll fail. During the encounter with the Red Bull, in solving the problem, Schmendrick’s magic creates many others. Namely, it sends the unicorn into an identity crisis where she nearly loses herself, her memory, and her purpose. For a while it seems he should have left her to the Bull, who at least who’ve allowed her to keep her form when he imprisoned her. What is the use in succeeding, it asks, if you have to change? Why not stay trapped, and the same, forever? Why not fall into the fake fantasy of an identity that never has to change or question its values, but rather can stay permanent forever?
Schmendrick’s magic works here because it creates story, and indeed raises necessary questions about the unicorn’s quest and decision to continue the quest at all. For all that Schmendrick saves the day, deus-ex-machina-style, it’s not a simple fix. It’s a fix that challenges the unicorn’s whole identity. In becoming a human, she has to engage with the impermanence and fleeting nature of humanity. She has to wonder what it is to feel love, to grow old, to die. She, who has always been at a distance, always removed in the woods, immortal and unconcerned, is brought close to the tragedy and triumph and roller coaster of existence. She has to decide if, after all the grief and love and banality, it’s worth saving the unicorns. If it’s worth, indeed, going on at all.
