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Traditionally a blood oath was when one would shed their own blood(Cut yourself or something) and offer it onto a altar or whatever they believed right(sometimes a religious action) and swore to uphold a certain task, no matter what. Also it was when a person(s) would shed blood, usually they'd cut their hand and then shake hands in agreement to a task they swear to uphold.

More modernly it just means you swear to do something no matter what, and since it's a blood oath, modernly it means hurting or even killing others in oath, thus 'blood' oath, meaning blood would be shed in promise to the task, usually a phrase only used by mobsters and crooks.
The great samuraiKenji Setsuna bowed before the altar and cut his hand, offering blood, swearing to protect his people under any condition in belief before his God.

The Prince and his loyal Knight cut their palms and then shook hands, promising each other in a blood oath they'd never split apart.

Officer Eliot of the Chicago PD & Officer Hunter of the Seattle PD made a blood oath that whatever happens, they will stop crime in their city, willing to do whatever it takes to keep this oath.

Kingpin Chuck Norris told a footman of his black market empire that if he did not go through with the hit, he'd kill his family. The footman then made a blood oath, swearing he would.
by Zure the Dark Being April 11, 2013
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