Note: I would like to give a special thanks to IsaiahLeIstya , whose comment about Cirque de Freak back when I was talking about the very first cliché in this editorial, "Being Human is a Bad Thing" helped create this very idea. They are quite clever and even have a page stock full of great original content. I recommend giving their fantasy-themed content a try.
Imagine yourself on the biggest, wildest roller coaster that you can think of. It can be an existing one such as El Toro from Six Flags Great Adventure or one from your own imagination, filled with everything that gives you that awesome adrenaline rush you desperately crave. You are on this chosen roller coaster, ready of the scariest and most thrilling time of your life. And for the first half of the ride that is exactly what happens. You are filled to the brim with glee as you circle multiple loops, upside down tracks, free falls hundreds of feet high, and the weightlessness you experience while securely fastened into your seat. But then out of nowhere the track become completely level and flat. The thrill you were once experiencing is now gone. The roller coaster has become a shell of its former self to the point you cannot even call it one anymore. This roller coaster, imaginary or not, is the metaphoric representation of one of the most story ruining elements; tonal shifting.
Tonal shifting is like a roller coaster but for the tone set for a book. Say your fantasy novel for example originally is set as a heart lifting tale of a rag-tag team of heroes taking down the evil dictatorship of the main villain. Although very basic, if done right an audience can be drawn to the story for multiple reasons with this established tone. The characters could be very complex and relatable, the stakes are high, the story is out-of-the-box imaginative, or it is just a nice slice of happiness needed in an age when most stories tend to go in a darker direction. Whatever the reason, the tone creates an established environment that we as readers expect to stay consistent. But then suddenly the happy, loose tone is changed to a dark environment with the murder of the fun loving sidekick halfway through the story. All of a sudden, everything the readers felt comfortable with was taken away. The shifted tone may have created more stakes for the heroes but at the cost of possibly the story itself. By the end of the book, everyone has died except the bad guy and tyranny rules free of restrain. The shifted tone has ruined the otherwise stable story, which lost its way of making the audience feel happy by ending on a different note than it began.
You have to always no matter what keep the intentions of your story at the same tone. If not, it could call for disaster. The death of L from the Death Note manga shifted the tone of the story to a less relatable tale about Light Yagami just trying to become more of an unlikable character. Black Butler's second season went off the original manga's intentions of creating a dark tale about young Ciel purging the Underworld of Britain in revenge for his murdered parents and instead made a bland story about two rich kids fighting over their demonic butlers. The Warriors book series after the second arc changed from an exciting tale about the secret lifestyles of outside cats in Britain to a cliché prophecy of three story that was not even half as strong. Complete tonal shifts are a true danger for any series. If done only for cheap shock value or to continue milking the story, tonal shifts are story destroying.
The only reason a tonal shift should ever be used is for the following; to improve the plot formula of a new series or to break the set basics of a specific type of story. Madoka Magica, both the anime and manga, is an example of a well done tonal shift that ended up making the series iconic. At first starting as a typical magical girl story fueled by the power of friendship, after the fight with witch Charlotte does the series take a sudden change to a serious tone. This ended up creating a stronger message on the series's message of never letting yourself fall into despair, even if the world is not always perfect. Additionally there is the Harry Potter book series that went from a surreal magical experience to an epic against good and evil. In those cases, tonal shifts were used wisely as the author grew with the material and understood the risks of taking such action. The same should be said for an inspiring fantasy writer out there. Know the story well before changing the tone.