Klaus Mikaelson had a daughter when he was human she was turned into a vampire with the rest of the family
When Klaus daggered the rest of the family he had promised his daughter that he would never do it to her
But when Klaus got Hayley pregnant and she was harassing the youngest Mikaelson she stood up for herself and almost killed Hayley and Klaus didn't like it so he did something terrible
He went back on his promise and put a dagger into her heart
When Elijah saw a coffin he had never seen before he opened it and was very disappointed in his brother and he took the dagger out
But something went terribly wrong and Klaus will regret it forever