Dvergr

Dvergr (黒小人) is a term used to refer to dimutive humanoid beings of Norse mythology. It is most popularly referred to as dwarf. In the Toaru Majutsu no Index universe, the magical nature of Dvergr is doubted, and are referred to as simply a different race of human beings instead of an altogether different group of beings.

Etymology
Dvergr is derived from the name for the dwarf, though the true origin of the word is contested.

The kanji (黒小人) can be translated as Black Dwarf, most likely referring to their dark skin, as exemplified by Marian Slingeneyer.

Appearance
Much of their appearance is as of yet unknown, although, one can presume due to their depictions in popular culture that they are swarthy and have statures shorter than that of an average human being.

Culture
Continuing with the doubts of their existence, Dvergr are apparently gifted in techniques in manufacturing metals that were unknown to Nordic cultures, and are apparently well suited to underground areas. Dvergr's territory is apparently of living human bodies, and in turn the god's or giant's territories is that of the dead. It is unknown if this extends to practical knowledge of the human anatomy or strictly regarding the bodies being used in magic.

As of November 10th of the current timeline, Dvergrs have apparently become extremely rare.

Toaru Majutsu no Index SS: Kanzaki
They are first mentioned during Kanzaki Kaori's investigation on Alfar, a girl who was designed to look like an elf of Norse legend. Here, Dvergrs or dwarves are referred to as apparently just a race who have metallurgy techniques unknown to the Nordic peoples. Alfar was also designed to apparently act like that of an elf, as having disdain for metal and the underground.

Toaru Majutsu no Index SS: Stiyl
Richard Brave believed that the The Alaskan Runes are nothing more than the key to the secrets of Dvergr's skill to create exceedingly powerful spiritual items. His objective was the annihilation of the Anglican Church in revenge in doing so would use the techniques of the Dvergr and pursue Lævateinn even further using that knowledge. Later Stiyl confronted Leivinia Birdway as to what they would do with the Epitaph and the Horoscope, she responded saying that they had no interest in Norse mythology at all, and the techniques of the Dvergr would be of no use to us even if we had them and did not want the techniques of the Dvergr to bring chaos to society.

Hawaii Invasion Arc
Marian Slingeneyer of GREMLIN, appears for the first time after the failed invasion Hawaii, talking to Othinus over the phone regarding their plans. Here, she refers to herself as a Dvergr, and shows craftsmanship by showing Mjölnir type construct to Cendrillon, as her creation. She later threatens an injured Cendrillon that she would turn her into some mundane object of her choosing, and shows of her crafting tools.

Baggage City Arc
Marian plays a major role during the arc, and her magic powers and the nature of the Dvergrs are more explored.