Question: What character from Norse mythology led an invasion force of fictional race of extraterrestrial shapeshifters in the main Marvel universe?

Context paragraphs:
Homo mermanus: Homo mermanus is a fictional race of gilled aquatic humanoids that has appeared in numerous comic book series published by Marvel Comics.  This race is best known as the people who live in the Atlantis of Marvel's shared universe, known as the Marvel Universe.  The most notable member of the race is Namor the Sub-Mariner.

Dark Reign (comics): "Dark Reign" is a 2008–2009 comic book branding used by Marvel Comics.  It deals with the aftermath of the "Secret Invasion" storyline, which led to a shift of power in the Marvel Universe toward Norman Osborn.  The title "Dark Reign" refers to Osborn's rise to national power and the ramifications thereof.  Joe Quesada, then-editor-in-chief of Marvel Comics, stated that "Dark Reign is not really an event, it's what's happening in the Marvel Universe."  He believes that "Dark Reign leads to an interesting place in the Marvel Universe.  I think you'll see a pulling back at the end of Dark Reign, but you'll understand at the end of it what we were trying to get to."

Ultimate Thor: In the Ultimate Marvel Universe, Thor (Thorleif Golmen) is a fictional superhero based on the Marvel Universe version of Thor.  Thor is the Asgardian God of Thunder based on the deity of the same name of Norse mythology and a founding member of the Ultimates.

Spider-Man Unlimited (comics): Spider-Man Unlimited is the title of three comic book series published by Marvel Comics.  The first series began in 1993 and was named in its indicia as Volume 1.  It was set in the main Marvel Universe.  The second series was based on the animated TV series "Spider-Man Unlimited", and was not set in the Marvel Universe.  Its indicia also called it Volume 1.  The third series, which began in 2004, did not have a volume number listed and was set in the main Marvel Universe.

Skrull: The Skrulls are a fictional race of extraterrestrial shapeshifters appearing in American comic books published by Marvel Comics.

Chitauri: The Chitauri are a fictional race of extraterrestrial shapeshifters that appear in publications by Marvel Comics, specifically in Ultimate Marvel.  They were created by Mark Millar and Bryan Hitch.  They were created for the Ultimate universe franchise in place of the existing Marvel Comics alien species, the Skrulls, which play a similar role in the franchise's mainstream continuity.  Later, Marvel would choose to distinguish between the Skrulls and Chitauri of the Ultimate universe.  The species would later be adapted to other media, notably appearing in the 2012 film "The Avengers" as an invasion force led by Loki and even in the main Marvel universe.

Loki (comics): Loki is a fictional character appearing in American comic books published by Marvel Comics.  He is the adoptive brother and often the enemy of Thor.  He is based on the being of the same name from Norse mythology.  The character first appeared historically in "Venus" No. 6 (August 1949) and in modern-day in "Journey into Mystery" No. 85 (October 1962).  The character was created by writer Stan Lee, scripter Larry Lieber, and penciller Jack Kirby.  Though normally portrayed as a supervillain, he is sometimes depicted as an antihero.

Mjölnir: In Norse mythology, Mjölnir (Old Norse: "Mjǫllnir" , ] ) is the hammer of Thor, a major Norse god associated with thunder.  Mjölnir is depicted in Norse mythology as one of the most fearsome and powerful weapons in existence, capable of leveling mountains.  In his account of Norse mythology, Snorri Sturluson relates how the hammer was made by the dwarven brothers Eitri and Brokkr, and how its characteristically short handle was due to a mishap during its manufacture.  Similar hammers (Ukonvasara) were a common symbol of the god of thunder in other North European mythologies.

List of cosmic entities in Marvel Comics: This is a list of prominent cosmic or mystic beings appearing in the fictional Marvel Universe, as defined by status and/or god-like power level.  Licensed or creator-owned characters (G.I. Joe, Groo the Wanderer, RoboCop, Star Trek, Star Wars, etc.) should not be included.  Neither do temporary power-ups through outside sources (e.g. the Red Skull possessing a cosmic cube, etc.).  Marvel Universe characters who exist in other dimensions, timelines, or times (such as Dormammu, or Kang the Conqueror) "are" included.  Company owned characters below are also included with reference.  Marvel has periodically published guides to their characters under the title Official Handbook of the Marvel Universe, a Marvel Comics Presents series and a series of Marvel Universe Cards.  "Epic Comics, Marvel UK, Marvel Knights, Marvel 2099, Razorline," and "Ultraverse" are a trademark of Marvel Publications.

Mythology of Teen Wolf: The mythology of the MTV supernatural action drama series "Teen Wolf" follows a teenager named Scott McCall (Tyler Posey), who is bitten by an Alpha werewolf and must cope with how it affects his life and the lives of those closest to him and Dylan O'Brien as "Stiles" Stilinski, Scott's best friend.  Most mythological elements in Teen Wolf relate to supernatural creatures and draws heavily from Greek mythology, Norse mythology, Japanese mythology, Aztec mythology, Native American beliefs and Celtic herbalism.  The show has created its own unique mythos centered around shapeshifters and their connections to one-another throughout the world.

Supporting paragraphs: Chitauri Loki (comics)
Answer: Loki