At the request of Winston Churchill, several Golden Age heroes joined forces to battle the Axis Powers in Invaders #1, retroactively set in late December 1941.
Although Captain America and the original Human Torch had second-thoughts about calling themselves the Invaders, proposing alternate names ranging from the Protectors to the Revenge Squadron, Sub-Mariner convinced the team to use the term that Churchill suggested.
Soon after they arrived in Europe, the Invaders encountered a mysterious woman with golden eyes and only a vague recollection of her past. But the woman soon recalled that she was a marooned extraterrestrial who had escaped from the clutches of Nazi villain Brain Drain (Invaders #2).
As an homage to the Richard Wagner opera, Brain Drain had called the extraterrestrial woman Brunnhilde. But her actual name was MCM-XLI (the Roman numeral MCMXLI translates to 1941), and she despised being objectified as a legendary valkyrie.
Lacking the willpower to escape on their own, three male extraterrestrials still remained under Brain Drain's mental control. They answered to the names Donar (god of thunder), Froh (god of lightning), and Loga (god of thunder).
Though set in the past, Invaders #1-2 were published the same months as Defenders #26-27 (August-September 1975). The superhero Valkyrie was well-established by this time, but her identity as the real Brunnhilde went unrevealed until Avengers Annual #11.