This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Children of the Red King
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 great-aunt who is also a powerful hypnotist and shape-shifter. Not to mention the Not Quite Dead Yorath...
Ezekiel Bloor counts as an In-Universe example. He's very old and looks like a living corpse, and his teeth are black and cracked. Charlie even makes a point of thinking that people of a similar age could look much better preserved. Then there's his living place at the Academy, which is dark and dusty and filled with cobwebs, having not changed at all in almost a century. He's also...disturbingly gleeful at the thought of seriously harming or getting rid of a group of teenagers and pre-teens, Charlie in particular.
Asa Pike and Cafall the Changer's endowment. If there is not a sufficient light source, they will automatically transform into beasts, Body Horror included. No exceptions. Coupled with that, neither of them are completely in their right minds when this happens - in the first book, Asa is even implied to maybe have killed at least one person in his beast form, and staying a beast for too long has a very bad effect on his sanity. His mother even refers to his endowment as more of an affliction than an endowment.