Vampire

Vampires (吸血鬼) are mysterious beings who surpass humanity and consume blood in the Toaru Majutsu no Index universe. According to Stiyl Magnus, the numbers of vampires in this world, their exact powers and their locations are all unknown. Only that they definitely 'exist', since something that can kill them, Deep Blood, exists.

Vampires are fearsome creatures to mages, as shown when Touma first heard it from Stiyl and asked him if he was serious, Stiyl replied by calling Touma 'a fortunate person' to be able to think he's joking about vampires. Their very existence is a source of intrigue and curiosity for the Magic Side, and are highly sought out by magicians. Conclusively proving their existence is even referred to by Stiyl Magnus as the wish of whole world.

Himegami Aisa is currently the only character shown in canon to have seen vampires and lived. Everyone else who has seen a vampire has died, according to the Magic Side's records.

Appearance
Unlike vampires in other fictions, the vampires in the series look and act like any normal human being, who can laugh, cry or get angry. The only known difference is they drink blood and are immortal.

Vampires, when drinking Aisa's blood, turn into ash, although it is unknown if this is how all vampires die or it is only limited to the effects of Aisa's Deep Blood.

Principles
According to the light novel, vampires are referred to as the descendants of, the brother of and son of , who was forced to wander the earth as punishment for the murder of his brother. There is no background in the Toaru Majutsu no Index universe as to why vampires are called Cain's descendants, despite having his descendants detailed fairly well in later accounts in the.

Deep Blood Arc
The magicians in the series have yet to ascertain their existence since they are like normal human beings, Himegami Aisa having Deep Blood who is the only person revealed to have ever met vampires as vampires die immediately after appearing before her. Since vampires are still people with emotions and the fact that people close to her have died as they were turned into vampires, Himegami Aisa has great disdain for her powers and desire not have someone die because of her blood as such that is the reason why she went to Academy City so that she can find a way to remove it. However the method could not be found as such she settled for a compromise, she allows Aureolus Izzard to use her to attract a vampire as he needed it to save Index, then at least her ability can be used to save someone instead of just killing.

After the defeat of Aureolus, Aisa is given a cross necklace by Necessarius to seal-off her Deep Blood. Vampires have not been mentioned since, and currently a vampire character has yet to make an actual appearance in the series.

Abilities
It is unknown what abilities vampires possess as all vampires who are revealed immediately die after coming into contact with someone who has Deep Blood. However Aureolus Izzard states that vampires are immortal as is the reason why he uses Himegami Aisa, so that he can capture one and have them turn Index into one. Since they're immortal they can store infinite information in their brains without their bodies breaking, as such Aureolus wanted this specific property of vampires to save Index, and stop the need for erasing her memories (he was unaware that the need for memory erasing was actually the result of a spell).

Furthermore, because of their immortality, alchemists sought out vampires for this specific attribute. If they can replicate the vampire's immortality, they would be able to finish the chant that would allow them to completely simulate the world in their mind, and allow them to will their thoughts into the real world, eventually allowing them to control it, Ars Magna.

Vampires are also immune to classical vampire weaknesses such as crosses and sunlight, and are referred to as something that can cause a nuclear level global calamity, much like Saints. It is also thought vampires have a unlimited amount of mana, therefore are unsurpassed in magical prowess and are the reason no one that ever saw them lived, since none can survive a magical battle with them. Also, Stiyl stated that if you cut out a vampire's heart and store it in a magic sword the vampire will still be alive, and the sword will become a living magical artifact.