Vampire

Vampires (吸血鬼) are mysterious beings who consume blood in the Toaru Majutsu no Index universe. According to Stiyl Magnus, the numbers of vampires in this world, their exact powers, 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 when Touma first heard it from Stiyl and asked if he's serious, Stiyl replied while gritting his teeth saying for Touma to be able to think he's joking about vampires then Touma is indeed a fortunate person.

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 Magic Side's records.

Appearance
Unlike vampires in other fiction, the vampires in the series look and act like any normal human being, who can laugh, cry or get angry. The only difference here is they still drink blood, are immortal and have unlimited magical power (the last one is purely speculation by the various magic cabals)

Abilities
It is unknown what abilities vampires posses 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 study how a vampire's brain works. Because vampires are immortal but can store millenias of information in their brains, he wanted to use the information that he will gain to help index expand her brains capacity so that a reset would never be needed. But unfortunately for him the resets were a lie and was only done to protect the secrets of the grimoires.

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, mages have theorized 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 weapon.

Toaru Majutsu no Index
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 revelead 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. Afterwards vampires have not been mentioned since, and currently a vampire character has yet to make an actual appearance in the series.