What character from Norse mythology led an invasion force of fictional race of extraterrestrial shapeshifters in the main Marvel universe?
Chitauri:  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):  He is based on the being of the same name from Norse mythology.
Loki