In the world of Toaru Majutsu no Index it is one of the creatures summoned through the spiritual item Brísingamen by Freyja, a GREMLIN magician. It appears in the GREMLIN invasion of Tokyo, and was summoned by her to do battle against Kamijou Touma.
The creature appears a boar made up of dark red thread. Initially, it was smaller than the giants that Freyja summoned, which was tall enough to hit the tunnel ceiling. This however changed after Freyja ordered the boar to eat them, allowing it to grow very large.
Like all of the summons of Freyja, she uses a jewel that she throws and then says a chant. The chant required to summon Hildisvíni is "Cost 1. White. Call / / Hildisvíni."
Of all the summons that Freyja has only Hildisvíni has been actually shown to eat the other summons under Freyja's command. This trait of eating other summons is shared with Níðhöggr Vol. 02 but it was only mentioned though the effects were still seen. Upon eating a summon, Freyja will say another chant like "Cost 1. White. Shift / / Hildisvíni!", with the cost incrementing for each summon eaten. For each summon it eats, it will add to its growth. The reason for this is that as Norse mythology is a mythology of mutual destruction, it has been set up from the beginning so that over 99% of the gods and their enemies would die fighting each other. With that as a basis, she constructed a spell that accentuates that fact, and create an attack that allows good and evil, white and black to devour each other. This may be the reason why it only eats "Black" summons, specifically the giant summons called Þrymr.
Like all of Freyja's summons, they are immune to pain, but can still be destroyed. Gravity and the laws of physics still affect the creatures, as such they can lose balance when a part of them is destroyed, or can crush metal if they are large enough.
- Shinyaku Toaru Majutsu no Index Light Novel Volume 08 Chapter 4 Part 2
- Shinyaku Toaru Majutsu no Index Light Novel Volume 08 Chapter 4 Part 16