Its the vow that has made the Mikaelsons who they are.
And in the season 3 finale, its the vow that damns them.
The Mikaelsons, however, still think hes dead.

Credit: Bob Mahoney/The CW
And for Hayley and Elijah, their grieving involves sexy morning-afters.
Vincent asks Marcel if hes going to be able to hold up his end of the deal.
Now that Vincent took care of the ancestors, can Marcel take care of the Mikaelsons?
Can he actually kill the people he once called family?
As far as Marcels concerned, Elijah killing him sealed the deal.
As he puts it, Whatever happens next, they earned it.
Oh, and they poison Freya because #war.
And it begins rather quickly.
Bringing up Davina a bad move Kol claims what Marcels doing is an insult to Davinas memory.
So … Marcel bites him.
As a fight breaks out amongst the Mikaelsons and Marcel, he then bites Elijah.
(NOOOOO!!!!!!!!)
Thankfully, Rebekah arrives Hayleys genius idea to stop Marcel to press pause on the bloodshed.
And shes ready for an explanation.
But as we all know, there is no cure.
Marcel informs her that Elijah and Kol are as good as dead, and Klaus will soon follow.
So what does that mean for Rebekah?
Marcel claims hed never hurt her, but he also refuses to back down.
But when Rebekah brings up the idea of justice, Marcel agrees that he can give her that.
And that means Klaus is going to be put on trial.
Furthermore, Elijah tells Klaus not to seek revenge on Marcel.
This was not Marcel.
This was me, Elijah says.
Hes talking about needing Elijahs help in life.
Theyre two scared brothers and nothing more, and thats what makes it so impactful.
The problem is that she needs time.
Also, she needs Klaus to stay alive.
He sees now that he was wrong.
He then asks Hayley to take care of Hope.
And with one last hug between Rebekah and Klaus, the trial begins.
Step 1: Testimonials.
But Klaus doesnt apologize.
She knows what Klaus has done, but how is it different from what theyre doing to him now?
The strong fight to determine who is strongest, she says.
They all fight for power.
Klaus instinct is to defend his blood, just as everyone in the room is looking to avenge theirs.
She asks, Would anyone here do any less?
But Marcels not buying it.
Alls fair in defense of the family is the very idea that got them here.
And after all, hasnt Klaus punished Rebekah more than anyone?
With the curse slowly taking over Rebekahs mind, shes easily influenced by Marcels words.
And just like that, Rebekah is ready to divulge the brutal truth about Klaus Mikaelson.
Switching gears, Rebekah decides that Klaus actions were never in defense of family.
He destroys everything he touches, she testifies.
After all, he murdered their mother and both of his fathers.
Hes daggered his siblings repeatedly.
She decides that he needs to suffer the way theyve all suffered because of his actions.
And that means its decision time.
I want you to be happy he tells her as he kisses her on the cheek.
Step 3: Back at the trial, its Klaus turn to speak.
First, he puts Davinas death on Marcel.
Shouldnt he have warned her about what happens to those who cross Klaus Mikaelson?
And as for everyone else in the room, so he killed their loved ones, what of it?
And that alone far offsets his crimes.
I owe you nothing, Klaus tells them.
So hes ready to die.
In the end, you will all know that I won.
You will remember me, Klaus Mikaelson, your maker.
Right on cue, Rebekah yells, Death is too good for him!
Instead, they decide to make him suffer.
Rebekah calls Hayley and tells her the plan worked.
(Her anger was all an act!)
Freya then does a linking spell that connects all the siblings and ties them to Klaus lifeforce.
As Rebekah tells Hayley with her final words, its up to Hayley to make Klaus sacrifice count.
But he knows that Hayley wont rest until their family is united.
You are the legacy this family has always desired, he writes to Hope.
The promise we fought to protect.
You are, and always will be, our hope.
And yet, Vincent doesnt seem too happy with Marcel.
And when one of the not-fully-dead vamps attacked Kinney, Vincent had seen enough.
As Rebekah tells Elijah, He knew he had to be anchor to Freyas spell.
It seems, for once, Klaus Mikaelson is the hero of this story.
Whats great about this finale is less about this final hour and more about this season as a whole.
And in doing so, it led Klaus to a moment of redemption years in the making.
The powerful finale was the culmination of everything set into motion throughout the masterfully built third season.
And the shows best story arc at once ended and seamlessly launched itself into the next season.
And I, for one, cannot wait.