The Donovan/Maxwell family has been at the heart of The Vampire Diaries' main plot this season, so it makes sense that Kelly and Vicki both made their triumphant returns to Mystic Falls in "We're Planning A June Wedding." Kelly made her return first, showing up as a last minute guest to Caroline and Stefan's last minute wedding. And, though at first it looked like the wedding had failed to bring Katherine out of hiding, it turns out Kelly was working for her all along. Kelly died two years ago, and has been in Hell ever since. And she's not the only Donovan family member who's back in town. Kelly and Vicki returned to The Vampire Diaries, and Katherine has plans for them.
Instead of showing herself, Katherine let Kelly and Vicki do her dirty work for her, promising them an eternity of peace and nothing (a dream compared to their eternity in Hell). Kelly, who, coincidentally, had to kill people to stay alive outside of Hell, was tasked with messing with the wedding while also getting close to Matt to get intel. Now that everyone knows Katherine is coming back, the Salvatore brothers have a plan: create a new dagger specifically to kill Katherine, which means making one out of her bones. After failing to get the dagger from Matt's dad before he gave it to Stefan, Kelly didn't interrupt the wedding. Instead, she waited until the reception to blow up an entire building and almost kill Bonnie and the twins.
She failed to kill anyone, but it turns out that wasn't even the plan. No, all she had to do was distract the Salvatores long enough for Vicki to make her way to the Maxwell bell. Katherine doesn't just want to destroy Stefan's wedding, he wants to destroy Stefan, Damon, Mystic Falls, and everyone in it — she wants to bring Hell Fire. And, to do that, she needs a member of the Maxwell blood line to ring the bell. The episode ended with Vicki reaching the bell and ringing it, with no one around to stop her.
We'll have to wait until next week to see if Damon and/or Caroline can run fast enough to stop Hell Fire from burning Mystic Falls to a crisp in the series finale of The Vampire Diaries.