Spell tags are a type of ofuda, a magically infused paper talisman wielded for both defensive and offensive purposes. These traditional magics were used predominantly by members of the Japanese Imperial Demon Army, and the Hyakuya Sect.
They were most commonly long pieces of brown or white paper of consistent size and length. The large writing on them was red in color and took up most of the paper.
Supportively, they have the ability to serve as a useful buffer against various magical forces and can lessen their effects, which made them able to reduce the effects, but not cure, the apocalypse virus, functioning as an effective supplement alongside technological counter measures for medical staff. They could also ward off the influence and effects of cursed gear, as shown by how many were placed on the doors to the demon weapon room hidden under Second Shibuya High School to prevent the effects of the cursed gear from expanding and likewise Sayuri Hanayori was able to use spell tags to diminish the effects of an activated black demon cursed gear. Spell tags can also be used as barriers to prevent passage, as Mahiru planted wards all over to block Yu and Mika's escape route, though Guren believed they could still get past them.
Though outclassed by cursed gear and other weaponry, spell tags could still be relied on for unconventional combat styles. Despite their small size, they have been shown to be capable of being used efficiently in battle.
Talismans can be applied into cursed weapon to augment it's power. Additionally, Both the Hīragi and Ichinose Family have curses that could be applied on the opponent's body by placing talismans on the weapon. Guren Ichinose made use of one that caused an explosion to unnaturally occur on the enemy's body for physical damage. Manipulating two at once, Sayuri was able to dispel and counter a vampire sword attack, with them flashing blue and disappearing with their magic having been used.
There also exist spell tags capable of releasing destructive explosions and could automatically attach themselves to the target and through magic would remain there, unable to be pulled off. They could apparently be detonated at the will of the placer, for instance Guren could cause one to explode during a training session with Yu for a distraction. Similarly the caster may exert some control over the impacts that a spell tag could have, such as the size of the explosion. When practising against Yu, the explosion did little more then produce a minor bang to illustrate that Guren had won the practice round. On the other hand, their possible full power was witnessed when Guren was fighting against Mika. One spell tag had the power to level a multiple story building and cause extensive collateral damage with its explosion. However, the spell tag's destructive power could be disenchanted, as Shinoa was about to overcome their power that way, and stronger vampires were capable of out speeding the explosion itself. Similarly their explosive effects could be defended against with a sufficiently powerful being with an equally proficient weapon, as Crowley Eusford demonstrated.
More unconventional forms of spell tags could also prove devastating such as illusions, as they could serve as an effective distraction and even have actual effect on the victim physically, such as copies that could affect the physical world. Illusions could be countered by superior illusions, as shown by how Mahiru easily repelled Kureto's illusions with more than 100 spell tags. Mahiru also could use shikigami like spell tags that allows her to dodge an attack if slashed.
Eventually the Hyakuya Sect would also gain possession of the spell tags, using them in similar manners as the JIDA. However they also developed certain techniques shown when they manipulate the spell tags using them on their chests.
In his fight to capture Mika and Yu, Guren was able to use a single large spell tag that could both resemble his physical appearance and convince Yu's Asura-kannon, infinite blades, knot of fate power that it was the real Guren for the swords to target and stab accordingly. Guren also used smaller spell tags that were attached Mika and Yu to release an energy to bind them.
Advanced users is also able to prepare a hidden traps using spell tags that if triggered it could be powerful enough to kill the enemy, and vice versa, as other users are able to counter it and disable it as seen by Guren's proficiency in both disabling traps using his own spell tags and combat at the same time.
With spell tags displayed, Saitō reveals how an illusion of himself breaking out of a prison, and skewering many vampires was done. The Hyakuya Sect's spell craft could create a Saitō capable of fighting against third progenitor Lest Karr and fifth progenitor Ky Luc, where it was capable of speech, using his chain weapons, and seeing from its perspective. After this Saitō lost a leg and appeared to be defeated, it clapped its hands together where it was seen how Saitō had never escaped, neither was the prison destroyed.
Using her cursed gear, Sayuri could produce a sword made from multiple spell tags combined together. These were strong enough to withstand lower ranked vampire weapons, and deal damage to them. Though able to hold its own against lesser vampire weapons, against first class weapons, like Mika's, the magic failed and the sword fell apart into now useless spell tags.