The Hybrid Curse

The Hybrid Curse was a curse placed on Niklaus Mikaelson by his mother, Esther, in an attempt to keep his werewolf side dormant and limit his power to that of an Original Vampire.
She placed the spell on Klaus shortly after he was turned into a vampire, when he made his first human kill and triggered his werewolf abilities. This event led to the revelation that Klaus was the product of an affair Esther had with a werewolf villager, Ansel, and was the world's first vampire-werewolf hybrid. This curse suppressed Klaus' werewolf side and kept him from being able to access any of the powers that came from being a true hybrid.
The curse was eventually lifted through the Sacrifice Ritual over a thousand years later by Greta Martin.