Hræsvelgr

Hræsvelgr (フレースヴェルグ) is a giant from Norse mythology who takes the form of an eagle. According to Index, it is a great eagle that devours the dead in Norse mythology. It is so large that flapping its wings creates all of the wind throughout the world.

In the world of Toaru Majutsu no Index it is one of the creatures summoned through the spiritual item Brísingamen by Freyja, a Gremlin (Norse) magician.

Appearance
It is described as a large living creature that resemble and eagle that was made up of a great amount of red thread. It is mentioned that they look like fighter planes that people could only see in movie. It is thirty meters long.

Principles
Despite being made out of thread it is capable of matching the speed of Academy City's fighter planes, most likely the HsF-00s. It can flap its wings to travel at acute angles with supersonic speeds. Using its wings, it can produce a spear-like gust of wind that even forces the fighter planes to dodge it and knocks its missiles out of the air.

Like all summons of the Brísingamen, 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. It is likely that the eagles are vulnerable to the Imagine Breaker.

It is unknown how Freyja summons the creature.

Magic God Othinus Arc
Hræsvelgr appears during the invasion of Tokyo. It's role was to make a security net around Tokyo Bay to prevent enemies from getting close to Sargasso. It may also have been summoned to destroy those from Tokyo who planned to escape through helicopters and chartered planes like the Muspells. It later engages Academy City fighter planes in battle.