The devil has come to Mystic Falls. While Bonnie and Caroline were busy going into Damon's mind to try to save him from his catatonic state in "Nostalgia's a Bitch," Stefan was making deals with sirens — Seline, to be exact. Spoiler alert: Stefan's plans fell apart just about as quickly as they came together, but his actions did have a very serious, unintended consequence: Cade. Cade is in Mystic Falls on The Vampire Diaries, and he isn't bringing anything good along with him.
After overhearing that Matt and Seline were planning on killing Sybil using the Mystic Falls Charter Bell, Stefan was ready to kill Matt and/or Seline to put an end to that plan. He wants Damon back so that they can go on serving Cade, something he cannot do if his brother is stuck under Sybil's spell. But, Seline is able to offer him something else: thousands of souls to deliver to Cade by midnight. All they need is for a Maxwell family member — either Matt or his father — to ring the bell 12 times, and Stefan will get his souls as Matt gets his revenge on Damon for killing Vicky and Tyler. The catch? Ringing the bell 12 times will open up a portal to hell and let in hellfire, which will destroy everything for miles, killing Sybil...and all those living in Mystic Falls.
In the end, Damon woke up when Stefan let slip his plans to consume Mystic Falls into hellfire and was able to stop a compelled Matt from ringing the Bell 12 times. However, Matt did ring it 11 times, creating enough of a rift to allow Cade passage back into Mystic Falls. The episode ended with Cade finding Sybil and Seline and consuming them in hellfire.
It's unclear whether or not Sybil and Seline are gone for good, but, whatever their fate, Cade's presence in Mystic Falls can only mean one thing: things are going to get worse before they get better.