In season 3's opener, Damon and Alaric are out searching for Stefan, who has disappeared with Klaus. They go to one house and find two young women sitting on the couch, covered in blood. Damon calmly identifies it as Stefan's work, saying "He feeds so hard he blacks out and rips the bodies apart, then feels remorse afterwards and puts them back together." When Alaric asks him what he means, Damon pushes one of the corpses with his foot, and her head promptly falls off.
The tomb full of mummified vampires. It's terrifying.
Silas' tomb. Espically when he awakens; first thing he does is gives off an ungodly roar, and violently suck Jeremy's blood, breaking his neck. And even though we don't see Silas in full body at that scene, what we do see is still terrifiying.
Klaus's attempted hybrids in "The Hybrid". They're all sweating and bleeding from the eyes before they go rabid.
Another one for Silas' power, when he first meets Klaus he stabs him in the back with a white oak stake and snaps a peice of it off inside of him. Not scary enough for you ? Okay then, how about the constant pain Klaus went through, trying to carve the peice out of his body, then heres the kicker - it turns out the impalement and pain where hallucinations caused by Silas. The fact Silas can make anyone feel pain thats not really there, Klaus and Caroline's reactions are spot on.
Stefan's fate in the season four finale. Stabbed by Silas, thrown into a vault that can only be opened from the outside, and tossed into the bottom of a deep lake. Bad enough for a human. Worse if you're a vampire that can't be killed by drowning. Meaning Stefan will spend God knows how long in a constant cycle of waking up, only to be immediately drown again. But that's not the worst part. The worst part is knowing that Silas is still out there, passing himself off as Stefan, fooling and manipulating every single person he cares about. And there's not a damn thing he can do about it.
Silas's main power, which makes him walking Paranoia Fuel. He can appear as anyone you know and love, so how the heck are you supposed to know who to trust? Or let into the house...?
In the season five premiere Silas publicly kills the Mayor in front of the townspeople, after nonchalantly revealing his nature, before compelling the whole crowd into not talking or making any movement, all while Bonnie watches.
The Flashback of Qetsiyah showing Silas the heart of his true love.
The cost of Bonnie being brought back to life. As the new anchor between the mortal world and supernatural Purgatory, every supernatural who dies passes through her body to reach the other side. Meaning, Bonnie feels their deaths. Every single one of them, and it is painful as fuck. Not only that, Bonnie like Amara before her won't be able to tell the difference between the dead and the living, and it's heavily implied that that was a heavy contribution to what made Amara turn insane.
Damon's five years as an Augustine vampire, tortured and mutilated every day while only subsisting on a ration of blood per day. Worse is that he's never told anyone about it, until Elena got dragged into it with him as well.
The worst part, of course, is when he eventually escaped. Damon made it his life's mission to kill off every single member of the Whitmore family, except for one child who would grow up, start a new family of his own....and then Damon would kill them in the same pattern over and over. Every single death that has happened in Aaron's life was because of Damon visiting his revenge on people who, in all likelihood, had no idea what his great-grandfather was doing.
Elena wakes up in a different part of the building and meets Enzo, the vampire Damon befriended while he was imprisoned. Enzo had been an Augustine vampire ten years before that...meaning he has spent nearly seventy years being tortured and mutilated on a daily basis.
And finally, there's the fact that Elena's father was one of the most revered members of the Augustine scientists. Meaning, in all likelihood, he participated in or was aware of what Dr. Whitmore was doing to the vampires. Imagine being Elena, and wondering if your parent would torture you for being a vampire. Is it any wonder she breaks down?
Oh, it gets better. Dr. Gilbert performed his experiments in the basement of his town clinic. A place where Elena could and did wander into as a child and hear the sounds of someone being electrocuted and screaming.
It turns out that there is a Hell in the TVD universe. And 1.15 shows us Katherine Pierce getting dragged there kicking and screaming into a black void.
And Markos's resurrection, which causes the Other Side to be destroyed, subjecting the supernatural spirits in it to the same fate.
In season 6, we meet Kai, who gleefully tells Bonnie and Damon of how he killed his siblings without remorse, and has the power to drain witches of their magic to use as his own.