In season 3 of The Originals the Mikaelsons were desperate to kill the Upgraded Original Vampire Lucien Castle. To finally kill him they used Davina's Ancestral Spirit to gain access to the magic that created him and after reversing the Immortality Spell they killed him.
But in season 5 of The Originals it was revealed that Klaus had secretly kept one, last stake.
Why didn't he tell his family about that? It could have prevented a lot of trouble, Lucien wasn't immune to the White Oak magic.