This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Magi – Labyrinth of Magic
Dungeons. To ordinary people in the Magi universe, dungeons are Eldritch Locationsof doom that hold nothing but death. Even after Sinbad showed that success was possible, thousands (and counting!) of people have met their maker just by stepping foot inside.
At least most people get a choice of whether or not they want to attempt a dungeon. Zagan's dungeon, however, forcibly pulls in anyone who wanders near. And then turns them into trees, as mentioned below, which become sustenancefor the dungeon monsters.
While mainly tragic, Hakuryuu's past is also horrifying. Watching his own brothers die horribly in front of him, with Hakuyuu wounding himself to cover Hakuryuu in his blood to shield the latter from the fire, and learning his own mother was responsible?That's a very pleasant memory. And even if it had his personality worsen and worsen, you can understand what made Hakuryuu so vengeful in the first place.
Jamil's gleeful expression as he tortures his slaves is certainly horrifying and comes across as a series of Moral Event Horizons for him.
The presence of slavery itself. In Magi, slavery is rampant. Though there do not seem to be major racial undertones in slavery other than the attempt at enslaving the Kouga girls and the attitude toward Fanalis, one can be forced into servitude for as little as outstanding debt, as in the case of Alibaba. Not only that, the only protagonist who really shows an aversion and hatred of slavery is Morgiana. Not even Alibaba or Aladdin cared much about the issue until they were influenced by her.
To be fair, at first, Aladdin just didn't understand slavery. He was the one who tried to free Morgiana in the first place.
What makes this even more horrifying is that the real ancient world wasn't any better.
Sinbad no Bouken has an entire arc discussing slavery as an industry. Some are picked out and treated well and elevated to notable positions or otherwise "invested in" to make specialized workers, others are moved through as quickly as possible for "general sale." Note that this includes children. The author did her research on this to a disturbing degree.
Fatima dropping the young and sick Nadja into the pit of ravenous dogs.
Zaynab's Red Mist, which had the power of influencing people's emotions to the point of hallucination and debilitating anger.
The missing people of Toran who got turned into trees in Zagan's dungeon.
When Morgiana activates her household vessel for the first time when defending the others from Zagan, she is quite powerful and quickly uses up her Magoi. While she's having an Imagine Spot about how useful this power is in helping everyone and how useful she herself has become to the party, her body is actually falling apart.It's notpretty.◊
The massacre of the Musta'sim Royal Family.
Young!Dunya screaming in terror when the soldiers find her and Isaac trying to escape.
The murder of Isaac, whose head was skewered through by swords.
Hakuryuu's arm falling off after Ithnan possesses him.
Al-Thamen's ultimate goal of breaking fate, which would result in anger, hatred, poverty, famine, civil strife, social upheaval, war, and other atrocities for all of humanity.
Alibaba's first fight in the Raem coliseum. He has his arm broken by a monstrous ape while the crowd cheers for it to eat him.
Hakuryuu's face when he speaks to his sister about all the hatred in the world. And his mother is just as creepy when they fight.
Chapter 211: The creepy unfocused eyes that Kougyoku displays when she is being controlled by Sinbad's Djinn Zepar. In the next chapter, we see how we get a visual representation of what Zepar actually does to his victims.
Sinbad's casual indifference at using Kougyoku as a unwitting spy against her family. When Alibaba threatens to tell her about what Sinbad is doing, Sinbad asks him what such a reveal would do to such an innocent person, revealing that he is fully aware of how horrible a thing it is he is doing.
Chapter 246: We see that having control over two Djinn enables Hakuryuu to do. He uses Belial to Mind Rape the soldiers with illusions until it becomes engraved on their mind and then uses plant circuits to push their bodies to the limit as mad bersekers.
And then we learn that just like Kougyoku, Hakuryuu was subjected to the effects of a secondary Djinn effect that causes him intense pain as long as he plans to kill his mother or the others, trying to forcibly restrain him. Kouen put that on him the moment after his father died so that he wouldn't try and act on it, which is why he never saw him as a threat.
Chapter 250: Hakuryuu beheads his mother. And that's just the start. He and Judar reveal just how far into depravity they've sunk, with Judar noting that Hakuryuu shifting his wrath to his own cousin, Ren Kouen, is only the beginning, hinting that the two of them very well may just end up killing everyone.
Fully confirmed in Chapter 254 where Hakuryuu states that anyone who has an opinion or thought different from his own is his enemy. Alibaba is disgusted and tells Hakuryuu that two people who completely think the same way doesn't exist, no matter how close they may be.
Chapter 257: Belial's metal vessel power is a scythe that cuts off all sensation in the body part it cuts, effectively "transporting" it to death. Hakuryuu's imagining of it is the sight of people strapped to the ground with the Grim Reaper looking down at them, about to let go of a guillotine blade.
In the following two chapters, we get to see Belial's Extreme Magic. It's a huge decaying dragon whose roar does the same thing as the scythe (but gradually) to anyone caught within its range.
Then, Alibaba keeps going anyway and clashes with Hakuryuu, burning both of his legs off. Yet, Hakuryuu starts laughing and saying he won... Because Alibaba got hit with Belial's scythe on his entire body, starting from the head. The result? Alibaba became an Empty Shell. And even Hakuryuu and Belial themselves don't know where they transported his spirit to, effectively killing him.
Koumei's strategy for dismantling Hakuryuu's magically enhanced and enraged forces is ghoulishly effective. It involves encircling them, sinking the ground around them, pouring oil, and setting it all on fire. The artist does not skimp on the horror of soldiers burning to death. Even though the strategy was to basically to put the majority of the forces in a massive sauna and sweat all the fight out of them without hurting them permanently, there was still collateral damage.
Chapter 276: The Reveal that Sinbad, one of the most if not the most powerful man in the world, is being influenced by the will of Il Ilah. Who turns out to be David, revealing that his scheme to merge with the mindless lump of power that was Il Ilah was successful.
Kouen eventually reveals who he learned his philosophy of eliminating war and strife in the world through expanding the Kou empire to the entire world: through Hakuryuu's father and brothers. This moment is after a young Kouen has a near complete mental breakdown encountering the scene of a massacre, where the decaying bodies of the dead were left impaled on and arranged around a massive tree - in what is likely a reference to the Tree of the Dead in 300.
That scene looks like it would belong more in a Junji Ito work than a shounen manga like this.
In Chapter 282, Arba has survived and found a new vessel through Hakuei. And Arba seems to plan on following Sinbad now knowing that Sinbad is David's vessel...
Chapter 295 reveals the Kou Empire had been working on an undead army to invade Reim with.