Klaus learns Rebekah's big secret on this episode:
In 1919, she and Marcel used Genevieve to do a spell that would bring Mikael back from the dead and do away with Klaus forever, so they could live happily ever after.
Rebekah then killed Genevieve and her nurse friend because she was afraid Klaus would find out she was behind the spell... only Genevieve's nurse friend was Celeste in a former life.
Hence why Celeste has now brought Genevieve back: to avenge their deaths against Rebekah's.
But Elijah swooped in at the end, stuck a dagger in Klaus' heart and let Rebekah escape death.