Kill 'Em All

Damon makes a deal with Alexandria wherein he will deliver them Bonnie for opening the vault provided they send some workforce to help him track down the long list of vampires. Matt realizes that 2 years back, he killed Penny while she was capturing Stefan, who came to Mystic Falls to pay respect to Liz Forbes - a thought that had been compelled away by Stefan under a false cover story of the car accident as Matt would get shattered as he and Penny had just gotten engaged. However, Matt still holds grudge against Stefan for having destroyed his, Penny's and hundreds of others' lives. Damon-Enzo, Matt-Stefan, Caroline-Alaric, and the men from Armory team up separately to finish off Rayna's list. Pills' effects start wearing off of Bonnie enabling her to practice magic again. She opens the vault and Alex finds Yvette's body in an unrecognizable state though hearing her voices. As Alex and the others search inside the vault, Bonnie traps them inside the building thus keeping her promise she made to Virginia. Finally, Rayna's last life gets transferred to Bonnie, but along with that gets transferred her hunter instincts, turning Bonnie, a girl whose entire life revolves around her near and dear ones who unfortunately are vampires, into a vampire huntress.

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