Blood Oath

Blood is a powerful symbolism, and sometimes even has mystical powers. The Blood Oath makes use of this to make a commitment that can't be broken. (Or if it is, it's crossing the Moral Event Horizon)

Bob has a secret he needs to get off his chest, but doesn't want it to be made public. Alternatively, he needs to be certain Al will do something for him. Either way, this is so serious that it requires making a blood oath. After all, once you're bound by blood, you can't break that promise, even if you're a villain.

In one common version, Al and Bob become Blood Brothers, cutting their hands open and shaking on the deal. The implication here is that Brothers would NEVER betray each other

In another, Al cuts his hand open, and simply lets the blood drip to the ground. This is sometimes highlighted by the phrasing of the oath: "If I break my word may the Earth drink my blood!" Then giving the Earth a 'taste' of your blood so that it knows what to look for if you fail to keep your oath. Sometimes this version is used to swear an oath of vengeance- and making it clear that you mean business.

If it uses blood and binds you to an oath, then it belongs here.

Can overlap with Couldn't Find a Pen, which is any kind of writing in blood. Contrast with Heroic Vow. See also Geas. May or may not involve actual Blood Magic. When someone has broken this vow so thoroughly they become ostracised they're The Oathbreaker.

Anime and Manga

 * It's implied that this is the traditional way in Mahou Sensei Negima to induce a Pactio. But that's messy and doesn't have nearly enough fanservice, so they kiss instead.
 * This is also all one really has to do to exchange mana in the Nasuverse as well. Again, sex is more fanservice.
 * In Naruto, an interesting variation occurs as Naruto swears to Neji on Hinata's blood during the Chunin exam preliminary battles for the final test, "You... are going down", promising to defeat him. He makes the same gesture at the beginning of their fight in the finals.
 * Summoning contracts are also typically sealed in blood, with a small blood offering made each time the summon is invoked.

Fan Works

 * Blood Oaths are Serious Business to celestial beings in Undocumented Features -- swearing by one's blood, especially if it's spilled at that moment, very literally binds the swearing god to do what they're promising to do. For example, early in The Symphony of the Sword, Utena accidentally prevents an enraged Corwin from making an unwise blood oath, and he thanks her for keeping him from doing something foolish.
 * In Drunkard's Walk V, Belldandy swears an oath upon her blood to never lie to, mislead or otherwise deceive Doug when she decides it is absolutely necessary for him to trust and believe her.

Film

 * Kevin Costner's Robin Hood does it when he swears to avenge his father's death.
 * The wild men swear to obey Saruman this way in The Lord of the Rings.
 * In the 1998 Merlin film, Merlin swears an oath, complete with drawing a small amount of his own blood, to not use his magic except to defeat Queen Mab.
 * Zach Galifianakis tries to do this in The Hangover. It gets voted down.
 * The Outlaw Josey Wales and Ten Bears take a blood oath to seal the "words of iron" peace treaty between the Comanches and Josey's friends at the Turner Ranch and Santo Rio. It is strongly implied, though unstated, that this also makes Josey and Ten Bears Blood Brothers.

Literature

 * Harry Potter has the Unbreakable Vow, which is similar to this, this taken to the extreme. Though the thing used to bind the two is magic, rather than blood, if you break it, you die.
 * It is revealed that before the events of the Gaunt's Ghosts series, Gaunt and several other officers made a blood oath with Warmaster Slaydo, promising to continue the Crusade and protect Saint Sabbat to the best of their efforts in the event that he died (which he did), and that this is the reason Gaunt is willing to disobey Lugo's orders.
 * Marie Brennan's Doppelganger duology features a blood-oath as a plot point in both books. The blood is technically used as a focus just to shape the spell, but breaking the oath does involve bleeding to death through the one wrist cut while swearing the oath, so it still counts.
 * In Tamora Pierce's Tortall universe, anyone who breaks an agreement that they signed with their own blood will die as their blood literally boils in their veins.
 * In The Baroque Cycle, the members of the Cabal swear various oaths of their loyalty to each other and specifically to Jack. Van Hoek swears by his right hand. When Jack reminds him "But Cap'n, you are left-handed!", van Hoek points out that he'll need his strong left hand to chop his right off if he ever breaks the oath—and then, as a symbol, he chops his own little finger off and throws it into the fire. (He then faints into the dust, but it's the symbolism that counts with these things.)

Live Action Television

 * In Farscape, when John needs Scorpius's help to rescue Aeryn, Scorpius first makes him perform a "Scarran blood vow", which involves both of them cutting their fingers and drinking each others' blood and then their own. John is understandably squicked.
 * In the last season of Angel, Angel signs a contract from Wolfram and Hart in his own blood. He learns this when the person offering him the pen stabs it into his hand.

Professional Wrestling

 * During a famous feud with Jerry Lawler in Memphis, legendarily crazy lumberjack wrestler Jos LeDuc famously took a blood oath (pronounced "oat") swearing he would end Jerry Lawler's career. LeDuc ramped up the Squick factor several notches by using a double-bladed axe and REALLY CUTTING A SCAR INTO HIS OWN ARM!

Tabletop Games

 * In Earthdawn, Magical blood charms are used to seal bargains: if either person violates the conditions, the magic punishes the violator.
 * Vampire: The Requiem has the Invictus (a covenant of fedualist vampires heavy on inherited power) and their Blood Oaths, magical bonds with heavy penalties for violation.
 * Changeling: The Lost has many forms of oaths, and your health or life can be put on the line for them. With heavy costs comes heavy rewards - the more you'll lose if you break the oath, the more benefit you can gain from it.
 * The card Sign in Blood from Magic: The Gathering. Apparently it's quite a long document, because it hurts as much as getting mauled by a bear.

Video Games

 * A quest chain in World of Warcraft starts off with you making one of these with a troll, to show that you'll both honor a truce. He honors it, in that he
 * In Fire Emblem: Radiant Dawn, a blood pact is a contract that two or more people sign in blood. If the terms of the pact are not upheld, the persons subjects suddenly die off . They can only be broken by
 * In Thief 2 Strange Bedfellows seal their alliance this way (blood on earth variant).

Webcomics

 * Madoc of Wake the Sleepers accepts a contract through blood on a bounty, as his crazy Assassin culture requires.
 * Rowasu of Juathuur is full of scars because of this trope.
 * In The Order of the Stick, a Blood Oath binds not just the oath-taker, but all of their descendants. And if they don't die trying to fulfill the oath, they're denied entry into the afterlife while their children have a go at it.
 * Of course, the specific instance mentioned here (namely, ) is an odd case. The father in question abandoned the oath pretty quickly, only to try to force it on his child when he was getting old. After the child's death (he died trying to fulfil it), he gets into Heaven while the father is stuck in the "waiting room". When the father gets enraged at this, the angel who made the decree chews out the father for being so careless with a blood oath and points out that the child didn't actually have to take up the oath, but doing so out of free will and trying his best to fulfil it makes him a better person than the father.
 * It should also be noted that "Start of Darkness" depicts taking the Oath much like getting a tattoo. Including the part where Eugene got it while he was smashed and had little recollection of doing so later.
 * In Bad Machinery Erin Winters demands that contracts acknowledging people owe her a favor be signed in blood. Given that the last time she was seen in Scary Go Round before reappearing without explanation in Bad Machinery she was in the company of and (technically) married to Aleister Crowley, and trapped in Hell, this may bode ill.