The opening sequence depicts a demon destroying a village, with fires spreading everywhere.
The Security Golems hidden in the Forest of Witches will actually kill intruders unless they act quickly, as Dominante and her party learn the hard way. Yikes!
The Diamond Kingdom's destruction of the Forest of Witches caught quite a few people off-guard.
The Witch Queen looks demonic as she uses her Blood Magic to manipulate Asta into killing her friends, while they are on scaffolds made of blood, nonetheless. The anime adaptation emphasizes this by making the sky darker and the ground glowing light blue.
If you interpret the threads she created as part of Vanessa's imagination while she's in the cage, it puts into perspective how much psychological damage has been done to the girl due to her mother locking her up in the cage.
When Vanessa's magic manifests itself in front of the Witch Queen, Vanessa can watch in horror as she sees Asta decapitating Noelle and then cutting down Finral in half.
When her plans to kill Noelle and Finral have been foiled, the Witch Queen use her Blood Magic on her own daughter as a last resort so that the Queen can obtain the Red Thread of Fate personally, giving a Slasher Smile in the process.
The deaths of the Elves at the hands of someone who's assumed to be a human using Light Magic.
The beginning part of Chapter 149. Rades, Valtos, and Sally get sacrificed as part of Patry's plan, and the sight is not pretty. Their bodies appear to be crumbling at first before their souls get forced out of their bodies, and afterwards, the bodies are blackened, as if they had been burned. The smile that Patry gives as he reveals his true self to them is truly creepy as well.
Langris' cold bloodlust against Finral, nearly blowing holes in the guy.