Nostalgia's a Bitch

Sybil has put Damon into a catatonic state, forcing Bonnie and Caroline to free her so that she will release him in exchange for the bell. However, Sybil realizes Damon's mind is in an induced hellish state, put there by himself because of his guilt. Bonnie and Caroline go into his subconscious to speak with Damon and tell him they forgive him for everything. Stefan kills Violet and reveals that Cade is requiring more souls so he makes a deal to acquire the bell for Sybil so that Damon will be conscious again and do his share of killing. Afterward, Seline offers another deal, which involves Matt ringing the bell twelve times in order to release hell on earth and kill everyone in Mystic Falls. Stefan compels Matt into doing so and then goes to retrieve Damon after being told that the Damon's need for forgiveness is about him. Once inside Damon's head, Stefan realizes Damon needs to forgive Stefan for turning him into a vampire all those years ago, causing every terrible thing that has happened since. Damon wakes up and Stefan is chained and told by Caroline that she loves him and she will do whatever it takes to restore his humanity. Damon stops Matt at the eleventh ring, which stops hell from burning the town, but allows enough power for Cade to come to the real world. In the final scene, Cade meets Sybil and Seline at a diner, where after telling the sirens that he'll take it from there, lights them on fire.

Plot summary from Wikipedia (CC BY-SA 4.0). Source