Favorite characters that make everybody scared shitless?

As the title states, who're some of your favorite cases of a character whose mere presence is bad news?

JoJo has many such examples. But probably the best is Jotaro. After the events of part 3, Jotaro is considered one of the strongest people on the planet. Most of the villains in part 4 have plans that boil down to "keep Jotaro as far away from me as possible." And in part 6, Pucci could only kick his plans off in full by setting up a very layered trap to get him out of the way.

Samus in Metroid. Prime 1 has many Pirate Logs that go "Okay, how can we use this new thing we discovered to try and kill Samus?" And Prime 2 has another log where they start panicking after finding out about Dark Samus, as now there's 2 of her running around. Metroid Fusion also shows you what it's like to be hunted down by Samus, and it's not fun.

Slade in Teen Titans. Never played for laughs, incredibly skilled, always scheming, always fucking creepy. Haunted is all about him nearly driving Robin insane from beyond the grave. And later, he comes back to life, this time with demonic super powers.