SPOILERS for all John Wick movies ahead.
The Blood Oath is one of the most important aspects of the John Wick mythology, with serious implications for John Wick: Chapter 2 and John Wick: Chapter 3 - Parabellum. From before the start of the first movie, John Wick is a legend, everyone knows his name, and they’re all wondering the same thing: is he back?
In John’s former life, he was an elite assassin and a part of a mysterious secret society. Retirement isn’t supposed to be an option, but after he met his wife, Helen, he was a changed man. He approached the crime lord Viggo Tarasov, who said he could retire if he completed an “impossible task.” A single night of murder and mayhem that would swiftly consolidate the power of Viggo’s competitors under his rule, making him one of the top crime lords in New York City.
Determined to leave his violent life behind, Wick seeks the aid of Santino D’Antonio, whose family sits in one of the 12 chairs of the elite “High Table” of global crime lords. John takes a blood oath to garner Santino’s assistance in completing his impossible task.
The term "blood oath" is commonly used in the real world to depict an unbreakable promise. Historically speaking, several cultures had various forms of blood oaths that involved the actual letting of blood, sometimes mixing it with a handshake as a sign of commitment. In the world of John Wick, it’s a little of both. When committing to a blood oath, the pledge presses a bloody thumb to the right half of a circular device called a marker. This is a commitment of blood, meaning the debtor is swearing to fulfill the oath with the blood symbolizing the penalty for breaking a blood oath.
After John seals his commitment to Santino, he gets to work eliminating Viggo’s competition. As Viggo would come to say in John Wick, “The bodies he buried that day laid the foundation of what we are now.”
Burying his guns and gold under a cement slab in his basement, John settles down with Helen for a little over five years before she tragically dies of an illness, arranging for a puppy to be sent to him with a note: "you still need something, someone, to love. So start with this, because the car doesn't count.” We all know the rest of the story. Dumb luck would lead Ioseph Tarasov to steal his car and kill his dog, forcing John out of retirement.
John goes to underworld assassin hotel, The Continental, to speak with the manager, Winston. Winston warns him: "Have you thought this through? I mean, chewed down to the bone? You got out once. You dip so much as a pinky back into this pond, you may well find something reaches out and drags you back into its depths." But this was personal. Ioseph had stolen his “opportunity to grieve unalone” when he killed his dog, so John was left to process his loss the only other way he knew how, and it involved the bodies of 77 mobsters, including Viggo and Ioseph.
Unfortunately, as Winston said, something reached out to grab John. Only a few days later, Santino shows up at his doorstep to redeem the blood oath. Santino wants John to kill his sister, Gianna, hoping to take the High Table seat she inherited from their father. He calls in the blood oath to accomplish this because it can be fulfilled without him drawing up a contract for assassination, leaving a paper trail.
When John refuses, telling Santino he’s retired, Santino burns his house to the ground. Knowing the only way to satisfy the blood oath is with blood, his or someone else’s, John acquiesces. After John’s completed his mission, Santino opens a contract on his head in a public gesture of vengeance, even though John satisfied the oath.
John comes after Santino, forcing him to flee to The Continental for protection, where Winston makes him press his thumb to the other half of John’s marker as well as in his log book to close out the blood oath. When John tracks him down, he shoots him in the head, violating the first rule of the Continental and forcing Winston to make him excommunicado, stripping him from access to all Continental facilities and amenities. Meanwhile, the Russian and Chinese seats on the High Table order a $14 million international contract on John Wick.
Winston gives John a blood oath marker and warns him he can only give him an hour’s head start before a world of assassins descend upon him. In John Wick: Chapter 2, there isn't any explicit explanation on this marker. It was assumed to be John’s original marker from his blood oath with Santino. In the first movie, John loses his wife and his dog. In Chapter Two, John loses his car, his house, and his phone, along with the last images and video of Helen. After killing Santino in the Continental, John is excommunicado, losing his access to the immense resources he could use before. With the blood oath satisfied, the marker serves as a reminder that John has gained ownership of himself once more. Even though he lost everything, he’s finally completely severed from his old life, owing nothing to anyone.
However, the blood oath returns in John Wick: Chapter 3 - Parabellum. Now on the run, John has to survive an increased price on his head from the High Table and no longer having access to Continental facilities. Although his blood oath connected to Santino is spent, he still has the one given to him by Winston. After escaping New York using a "ticket", a cross that connects to his upbringing, he travels to Casablanca. There he meets Sofia (Halle Berry), a fellow dog lover whom he has an open blood oath with.
This blood oath is a result of John helping Sofia hide her daughter from her enemies, presumably because she was being actively hunted at some point. John cashes it in to have Sofia take her to Berrada (Jerome Flynn), who in turn leads him to High Table overseer The Elder (Saïd Taghmaoui). Once Berrada is dead (killed by Sofia for shooting one of her dogs), this blood oath is over.