Tarnkappe

A Tarnkappe (タルンカッペ) is a spiritual item based on a cloak of invisibility which appears in Norse Mythology.

Principles
The Tarnkappe originally would have been a large cloak, which would enhance its wearer's strength and make their body invisible.

In the case of the Tarnkappe used by Alfar, it was analyzed and its workings reconstructed to turn it into a spiritual item in the form of narrow threads. These threads were laid out in a passageway, creating a magical screen like the film of a soap bubble and erasing a person's form from the background, leaving virtually no trace of them visible even to the enhanced eyesight of a Saint like Kanzaki Kaori, though she was able to notice the threads after focusing more.

Toaru Majutsu no Index SS: Kanzaki
During the events in Brick Wharf, Alfar initially hides herself, and an animal distorted by her power, from Kanzaki Kaori using a Tarnkappe made from silk threads.