Most of the evil endowed talents are pretty scary.
Manfred's however takes the cake. At just nine years of age he was able to erase the memories of a grown man and render him utterly helpless for ten years.
The reveal that Titania Tilpin is a witch is pretty horrifying, as well as the fact that she bears Lyell a grudge for rejecting her for Amy Jones and will do anything to make sure he doesn't wake up from his trance. It's chilling to see how cold-hearted and determined she is to have her revenge.
Dagbert Endless could be extremely creepy from the beginning, but the part where he drowns Tancred really takes the cake. The description of the scene combined with the fact that poor Emma could do nothing but hide as a bird and watch or risk sharing his fate makes it a bit of a tearjerker as well. It's worth noting that despite his later heel face turn, and the fact that the drowning wasn't permanent thanks to the Flames, Dagbert was still probably the only villain who succeeded at killing one of the main heroes in cold blood, despite many others trying.
When Yolanda corrupted her four nieces, they chose to stay in Yewbeam Castle while James and Paton left. Since Lyell was only two at the time and Grizelda's responsibility, he would have stayed with her in Yewbeam Castle, a place that still evokes bad memories for even Paton. Imagine a kid growing up in a dark, gloomy castle like that with a brainwashed mother, three brainwashed aunts and a Complete Monster great-aunt who is also a powerful hypnotist and shape-shifter. Not to mention the Not Quite Dead Yorath...