Finally, I got the Dark Ending on my 5th playthrough. It’s a troublesome-as-fuck ending that’s not worth it as it branches off in Chapter 13 and lasts 3 chapters. To open up the branch to the Dark route, you have to have obtained the True ending first, and then pick enough bad conversation options. There’s an invisible karma meter going on, and the easiest way to guarantee the opening of the Dark route is by repeating Chapters 1-3 because they have the most conversation options, and repeatedly act like a jackass. The branch will eventually appear at Chapter 13.
Spoilers below, for people who don’t want to bother with the Dark route.
In the Dark route, you basically become an unsympathetic asshole who wants to see the world engulfed in darkness for shits and giggles. You kill Cynthia and Gaston so they can’t retrieve the Holy Sword to free Adelaide’s spirit. Then you kill Yuri and Berkut so they don’t stop Veda from releasing the Corrupted Soul. Veda dies from being unable to control it, and it spreads across the world, overrunning it with monsters. You end up at the location of the seal (forgot name), where Jester thanks you for making his job easier. Toren and Fiona arrive to stop you two, so Jester tells you to fend them off as he goes to unseal the Dark One. Joined by Dark Adelaide and Dark Darius, you kill your former master and Jester unleashes the seal. You kill him too, because why not, and Dark Darius seems to act completely fine and normal here. He realizes that your goal was simply to plunge the world into darkness and have no angsty backstory unlike even Jester, so he attacks you because now there is no way to free Adelaide’s soul.
The seal is undone, so otherworldly forces invade the human world and you are the first to get killed off because you were standing next to the damn thing. However, you are satisfied because you got to see the world end and shit.
tl;dr: You cause the destruction of the world for shits and giggles, and you basically get to fuck everything up with a manic smile as all your former friends get killed by you in shock and despair. Everyone basically goes “WHY ARE YOU DOING THIS, WHAT THE FUCK IS WRONG WITH YOU!?”
It sucks that you have to complete the True Ending before you can see this, and it’s easily the least rewarding route in the whole game and totally not worth your effort. What is this measly 3 chapter branch? If you’re going to make the MC become an evil maniac, might as well give me a high-level battle with some of those dark forces that invade the world or something, and let me beat the shit out of all the gods and become the new ruler of the world. Or give me hot demon girls as a reward. Yeah, I guess I’ll just pretend this never happened. I like FABULOUS FRIENDSHIP just fine.