Throughout The Vampire Diaries Series
In Know Thy Enemy, Maddox kidnapped Katherine and took her to a Alaric's apartment. She woke up to find Maddox casting a spell on Alaric, who was unconscious and sitting on a chair. When Maddox finished the spell, Alaric stood up and walked over to her. When he said, "Zdravei, Katerina," Katherine realized that Klaus was possessing him.
In The Last Dance, Klaus taught history class at school, while pretending to be Alaric in Alaric's body. He saw Elena, the doppelgänger he needed to break his hybrid curse, and keeps a close eye on her through compelling various students. He showed up at the 60's Decade Dance, still possessing Alaric's body, and attempted to kill Bonnie by provoking her into using too much power. She cast a spell on herself that made her appear to be dead in order to trick Klaus.
In Klaus, Greta, Maddox and a few other people (presumably witches), brought out a box. Alaric stood helplessly as the witches performed the spell to put Klaus back in his own body. His eyes were closed, and when he opened them, he saw Katherine. Believing her to be Elena, Alaric whispered, "Elena?" before he collapsed onto the ground. Klaus then awoke in his own body.
In The Last Day, Klaus went to the bar and approached Alaric and Damon. Klaus thanked Alaric for the use of his body by saying, "Thanks for the loaner, mate".
In All My Children, Ric is at the Mystic Grill with Meredith when Klaus arrives with his brother, Kol. Ric keeps an eye on the Originals, letting the others know when to send Caroline in to make a distraction.
In Heart of Darkness, Stefan tries to get Alaric's darker persona to reveal the location of the last white oak stake, which he had hidden. Klaus arrives, snapping Ric's neck in an attempt to kickstart his darker side by killing him. Ric's alternate personality does reveal itself.
In Before Sunset, after Alaric had been transformed into an Original by Esther in her plan to have him hunt down her children. Ric captures Caroline in an attempt to lure the town's vampires to him. Klaus and the others devise a plan to desiccate Ric with Bonnie's help but they need to make a blood connection. They engage him at the school where Klaus, putting up the best resistance against Ric, manages to shove his fingers into Ric's chest, making the blood connection needed. However, Ric overpowers Klaus, nearly killing with the white oak stake before Elena arrives and Klaus flees with her.
In The Departed, after Klaus had been desiccated by Bonnie's spell, Ric desperately searches for his body, which he knows Elena and the others have. He recruits Jeremy into his scheme, finding that Klaus' body is at a storage facility. Fighting past Rebekah and Damon, Ric opens up Klaus' coffin and stabs him with the white oak stake, setting him ablaze and apparently killing him, shutting the coffin. It seems he's killed Klaus until it is revealed that Klaus body-jumped into Tyler's body. Ric was killed soon after when Elena passed away.
- Klaus likely possessed Alaric to get the others (Damon, Stefan, Elena, Jeremy, Bonnie, Caroline) off his trail so he would have a chance at kidnapping Elena so he could use her in the ritual and sacrifice her to break the curse.
- Klaus' spirit possessed Alaric while his body remained still, lacking Klaus' spirit inside of it.
- In Moonlight on the Bayou, Klaus now knows about Alaric coming back alive and has a family with Caroline and the twins.