Those with teraphobia (fear of monsters) will be caught off-guard from the many times a monster or pack of monsters hunt down the protagonists.
Picturing the demise of the unfortunate adventurers. They enter the labyrinth to fight against monsters that are an easy rank for them. But once they go into the lower levels, they will encounter even more bloodthirsty monsters such as minotaurs or goliaths, and they have to either Run or Die or face them head on. And it's too late to escape and no one's coming to save you deep in the labyrinth.
Kanu's rather well-deserved Karmic Death at the hands of Ottar's Minotaur is a lot more drawn out and detailed in the novel compared to the anime.
The Goliath. The writer dropped a Titan on the heroes, there is literally no other way to describe it: a towering, unstoppable humanoid from the deepest depths of the Uncanny Valley, with screams and roars that will stick with the viewer for hours and a Healing Factor that can shake off a barrage of attacks that could level a city block. We even get a lovely view of its jaw falling off and its skull after the aforementioned onslaught blew its face off, as the monstrosity gets right back up as if nothing happened.
Phryne in Volume 7, especially in the anime which holds nothing back. Not only is she utterly hideous, while she honestly believes she's the most beautiful woman, ever, but should she get her hands on a man she finds "tasty," she drags him off to a torture dungeon, pumps him full of aphrodisiacs, and then tortures him into a bloody, broken doll before she gets bored and throws him out, too traumatized to ever develop any kind of romantic attachment, ever again. Just the mere mention of the threat is so horrific Bell, of all people, is a crying, shaking mess, when Haruhime breaks him out, as Phryne is going to get some "medicine."
From volume 13: The Juggernaut. Sheer, unbridled, nightmare fuel.
In the beginning of the spin-off, Loki Familia finds themselves fighting an unknown and dangerous breed of monster, capable of melting weapons and overall, being difficult to beat. Swarms of them were even able to beat a powerful dragon monster, and these things attack even other monsters.
The first member of the group behind the monster attacks to be introduced is a high-levelled adventurer capable of killing Level 4 adventurers quickly and easily. She's also a tamer that can control the monsters that gave Loki familia so much trouble. And to top it all of, she's wearing the face of her victim just to disguise herself.
Loki Familia reaches the 59th floor, and are shocked by what they find. A monster that was once a Spirit, and the new breed of monsters they were dealing with were parts of it sent out to gather magic stones and make it more powerful. And in just two spells casted by the monster, Loki Familia suffered almost total defeat. Only a combination of Finn's top-class leadership capabilities, inspiration from witnessing Bell's fight with the Minotaur, and the fact that a lot of the people present were first class adventurers allowed them to come out of that battle alive.
Tione and Tiona's backstory. They were born into Kali Familia and were given Falna on their day of birth. They were then subject to Kali's program of Blood Knight-conditioning, which involves frequent duels...among their kin. As toddlers. One thing about how scary this can get: both of them got their first Level Up at the age of five. By killing one of their roommates each.
In volume 12, Filvis ripped open her chest to show Lefiya her magic stone, asking her that seeing her like this, would Lefiya still think her beautiful?
Another one is when Filvis revealed that shes stronger than Revis, the most powerful antagonist audience had seen in the series, then decimated the whole team in a blink of an eye, beating a lv.6 like it was child play. Absolutely horrifying.
Filvis whole backstory reveal in SO volume 12. She first witness the death of her familias captain, died herself, then got brought back to life by some weird spirits. She cant even kill herself to end the pain.
And those adventurers that got killed teaming up with her? She killed them herself under Eynos order.
And then theres Filvis little speech when struggling to think of a way to deal with Lefiya, at one point she was delighted by the thought of corrupting the younger Elf, wanting to see the once innocent girl fall into despair like she did.