The relationship between Klaus and Esther. It has been rocky, but got better from Klaus's side as he really thought his mother forgave him for everything he has done, when in reality, she was planning the death of him and his siblings.
Esther was Klaus's mother and she had an affair with a werewolf in the village they lived in resulting in Klaus's birth. For a long time Esther never told Klaus of his werewolf heritage until he became a vampire and activated his werewolf gene after his first kill. After Mikael, Esther's husband and Klaus's step-father, found out about this, he killed his wife's lover and his entire family. Esther cast a spell on him that ensured his werewolf side would remain dormant. After she rejected him as a son Klaus killed his mother and framed Mikael for the act.
Unknown to anyone, Esther's friend and mentor, Ayana, preserved Esther's body with a spell, and Klaus carried his mother's body with him for a thousand years.
In Bringing Out The Dead, Esther was awakened by Bonnie and her mother Abby. Esther then sought out her children and told Klaus she forgave him for what he did to her. Esther told her children that she wanted them to become a family again.
In Dangerous Liaisons, Esther revealed her true intentions to Elena - she wanted to kill all of her children because they are an abomination. Earlier in the episode, she convinced Klaus to come to the ball she was hosting and asked him to invite someone, which he eventually did, Caroline.
Klaus and his siblings found out about their mothers plan in All My Children. Klaus, Elijah and Kol confronted their mother and Finn (who was assisting her) and asked why she was prepared to kill her own children. She replied that for a thousand years, she was forced to feel the pain and suffering of their victims and they were a curse on the earth. Klaus told his mother to stop what she was doing, or he would send her back to hell. When Damon killed Abby, (Esther was drawing the Bennett witches' powers) the connection was broken and Esther fled with Finn.
In Heart of Darkness, Esther returned to see Rebekah. When she took over her body, the shell of Esther's body fell to the ground and Klaus entered, seeing her body. He showed little emotion even after he was told she was dead.
Klaus: (To Esther) "I just don`t understand. I killed you, and still you forgive me."
Esther: (To Klaus) "Forgiveness is not a chore, it's a gift."
Klaus: (To Esther) "Rebekah wasn`t even out of her box a day before she tried to ruin my life. What happened to peace, acceptance, family?"
Esther: (To Klaus) "You put daggers in their hearts. You want them to go down on their knees and kiss your feet for reuniting them?"
Klaus: (To Esther) "So it`s a crime to want our family to be as we were?"
Esther: (To Klaus) "You need to give it time, Niklaus. I`ve had a thousand years on the other side to be angry and to heal. I`m here to make sure this family does the same."
- Esther is, along with Mikael, one of the two beings that Klaus truly feared.
- Both such beings were his parents