This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Dragon Ball GT
Baby. Basically a more family-friendly version of the Thing, Baby is a supremely vindictive mutant parasite that infects all but a tiny handful of people on Earth, taking complete control over them, and when he picks a main host body his power warps it to completely unrecognizable levels while enhancing its abilities. Even worse was that in his earliest stages instead of controlling people he infected he ate them, wiping out entire spaceships full of passengers.
Shenlong's Evil Counterpart. Just when it seems like all the character's problems can be fixed once again by the dragonballs, they see a dark, menacing version of Shenlong who promises that things will get worse before they get better.