This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / My Little Portal
Rainbow Dash's cybernetic transformation. In the first episode, she gets badly injured in a confrontation against DISCoRD that leaves the walls and floors splattered with her blood. Then, in The Teaser of episode 2, she's shown with tubes implanted in her body, her eyes removed, and her legs replaced with mechanical prosthetics; the sight of her grotesque condition is enough to make Nurse Redheart comment that she looks even worse than when she was brought in. The worst part? The anesthetics given to her by the doctors barely had any effect, meaning she's not only conscious throughout the whole operation, but she can feel every second of it. And in The Teaser for episode 3, we're treated to a couple of second-long flashbacks of her screaming and writhing in agony as her new eyes are being added, shown on top of her surveying her new metal-clad form for the first time, on the verge of snapping completely... Cue happy opening titles!
Pretty much all of episode 5, namely the flashback of the fall of Canterlot. About the first thing that happens is Fluttershy being bloodily attacked, paralyzed and nearly eviscerated by a changeling. Much of the rest is just changelings and ponies very gruesomely killing each other off, including Chrysalis basically Force-chokingher subordinate to death for failing to take Canterlot Castle, literally ripping out Shining Armor's heart, and snapping off Twilight's wings. We also get additional doses of Rainbow Dash's previously described Body Horroras she is repeatedly injured and self-repairs during the battle. That Content Warning at the beginning? It's not bucking kidding this time.