Caroline, Bonnie, and Damon took turns entering Damon's subconscious on The Vampire Diaries in an attempt to shake him from his catatonic state. After confronting Sybil and demanding he get his emotions back, Damon was stuck in his own mind in "Nostalgia's a Bitch," with no way out. In an attempt to get him back, Caroline and Bonnie went into Damon's mind. And, lucky for us, Tyler, Grams, Sheriff Forbes, and Vicky returned to The Vampire Diaries as part of Damon's subconscious.
Turns out, Damon's way of dealing with all the guilt of having his emotions back on was hiding deep inside his mind in a world where he was never turned. And, when Caroline and Bonnie make a deal with Sybil to get her to lead them into Damon's mind, they find that a world without Damon as a vampire is one in which all their loved ones are still alive. As they venture into Damon's imagined Mystic Falls, Vicky greets them at the Grill, where Caroline reunites with her mother, Sheriff Forbes, and forgives Damon for everything he's done because he was there for her when she was dying. Bonnie, meanwhile, finds Grams who helps lead her to the Salvatore mausoleum, where she finds Tyler, Damon's most recent victim.
Speaking with Tyler, Bonnie realizes that Damon doesn't need Caroline or her forgiveness to wake up, he needs to forgive Stefan for turning him in the first place. Swayed into helping them wake Damon, Stefan enters into Damon's mind and gets much more than he bargained for when Damon forgives him. Damon tells Stefan that he is forgiven for turning him, and, thus, for setting into motion the death and destruction vampire Damon is responsible for. Stefan doesn't want to hear it, but Damon woke up free of Sybil's control and ready to accept his guilt and begin atoning.
Damon's officially back, and he's ready to fight for Bonnie, Caroline, and the rest of Mystic Falls. He even apologized to Matt for turning Vicky all those years ago. It doesn't make up for everything that he's done, but it's a start.