This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Digimon Frontier
The climax of the Mercurymon arc has Mercurymon nearly stuffing Takuya into a coffin and killing him.
As hilarious as his voice acting is, there's just something chilling about how playfully murderous IceDevimon is. Even worse, his dialogue implies that those... things in the basement of Cherubimon's castle were the result of him feeding on the data of Digimon that he captured.
Duskmon is actually a pretty brutal Deconstruction of the Adventure series. Take a random human child with strong emotions, drag him into your world against his will, and make him fight for you.
Lucemon's voice actor in the English dub voiced Izzy in season 1 and 2. For anyone that grew up with those two seasons, they'll recognize that voice anywhere when they hear it in any other character in the later shows. But with Lucemon, they go even further, giving him a more evil tone that's very disturbing. You're basically listening to an evil Izzy at this point and he's ready to destroy the world.