Thor is a Norse deity, and multiple versions of the character have been mainstays of the Avengers. This is the Joint Venture Universe's incarnation of the two primary claimants to the name.
Pride Goeth Before...
For thousands of years, Thor Odinson thought he knew his role in the cosmic scheme of things. His people, the Aesir - indeed, everyone on his home realm of Asgard - lived their lives cyclically, being born, living their lives of miraculous feats, epic feasting, and raging battle, then dying spectacularly in the apocalypse known as Ragnarok and starting the whole thing over again. This all changed the day his father Odin brought someone new into the cycle - a baby brother for the then-young Thor, a new child of the Aesir, though from where, both he and Thor's mother refused to say. Regardless, Thor embraced his new brother - Loki - and the two became inseparable, Loki's keen gift for mischief and Thor's boundless thirst for adventure leading them into no end of trouble as children and youths.
Finally, as adults, their combined talent for troublemaking went too far. Loki led Thor and his childhood friends, Sif and the Warriors Three, to Jotunheim, home of the Frost Giants, to steal the Casket of Endless Winter, the prize and pride of the Jotun people. They were met with strong resistance, and despite Thor's might, Loki's magic, and the valor of their friends, they were nearly slaughtered until Heimdall, Watcher of the Realms, warned Odin of his children's plight and the Allfather rode to their rescue.
Odin was mightily displeased by his sons' effrontery. He had brokered a peace with the Jotun King Laufey, a peace sealed by the trading of their ultimate gifts: an infant child from each sovereign's line, to be raised as the other's own. Hela, the Goddess of Death and the Jotun's finest warrior, was actually Thor's blood sister, and Loki, the youngest Prince of Asgard, was the only blood son of King Laufey. The sacrifice both monarchs had made, and the delicate peace that sacrifice had bought, was now threatened by Thor and Loki's foolhardiness and ambition.
In wrath and rage, Odin stripped Thor of his power, his title, his armor, and all that came with being an Aesir and cast him to Earth as a mortal, wiping his memory and leaving him with nothing but a new name: Donald Blake.
Intending for Thor to learn and grow from this experience, Odin also fashioned a key for Thor to regain his power: the sacred hammer of Mjolnir, forged by the Dwarves of Nidavelir, in the heart of a star frozen in time at the moment of its death. This hammer possessed all the powers that Thor had, and some that he did not, so Odin enchanted it so that only someone truly worthy of Thor's power - someone brave, kind, compassionate, and selfless, but with a warrior's heart and a strong fighting spirit - could wield its might.
Ignorant of his past, but aware that he had lost something precious, Donald Blake was nothing in the eyes of the world: yet another homeless man with no true purpose in life. However, as he went from shelter to shelter, making friends and losing them, watching people grow sick and die while powerless to stop it, his heart was fillled with regret and sympathy. This motivated him to begin studying, and studying hard. Eventually, he graduated medical school and opened up a clinic in lower Manhattan, near the F.E.A.S.T. homeless shelter that had first taken him in eight years previous, offering affordable health care and turning no one away who needed his help. It was during this time that he met Jane Foster, and the two fell in love.
Regrettably, even though Donald didn't remember his past, it would still come back to haunt both him and Jane - specifically, when Thor's old enemy Amora the Enchantress drove a wedge between the two by putting Jane in a life-threatening situation and having it be James Kincaid, a scientist of Jane and Donald's mutual acquaintance, who helped save her life.
The plan worked flawlessly; within a month, Jane left Donald and her job at the clinic, and married James, pursuing a second career as his assistant, and eventually his partner in the exploratory sciences.
At Amora's Mercy
Alone and despondent, Donald still tried to maintain his clinic, but grew frustrated, impatient, and restless. More and more often, he was haunted by dreams of worlds he had never seen, a life he had never lived, one full of battles, gods, magic, and glory. Simultaneously, he was being drawn ever deeper into the orbit of a strange woman named Carmen Ravenhurst. This turned out to be Amora, in disguise as a mortal, moving in to take back what she viewed as her rightful property. Amora worked dark sorceries to try and turn Donald back into his original self, into Thor, but a Thor under her complete control, to be her consort and henchman for all of their immortal lives.
Donald was spared this fate only due to divine intervention. Karnilla, the Queen of the Norns and a Goddess of Destiny, undid Amora's enchantments and wiped Donald's mind of all his memories of the mysterious Carmen. The Fates' reasons are theirs alone to know, but this interference sparked a long-time enmity between Amora and Karnilla that continues to this day.
Donald Blake, Archaeologist
Despite Karnilla wiping his mind clean of any memory of Amora or her alter ego, Donald still had dreams of being Thor, and these dreams became more intense and more frequent as time went on. Unbeknownst to him, the divine magic of his father Odin was weakening as the time grew nearer for him to enter the Odinsleep, a period of rest snd rejuvenation the Allfather was forced to enter every few years in order to maintain his power and longevity.
,Among the memories came an in-depth knowwledge of the mysterious substance called Uru metal, a powerful metal that was rare even on Asgard, and all but unheard of on Midgard. Thus, when a meteorite crashed to Earth, the international peacekeeping organization known as SHIELD sought out experts on the matter. Donald, knowing more about it than most, left his clinic in the hands of some local medical graduates he had once interned, and set off from New York to Norway to serve as one of the scientists examining the strange otherworldly substance.
The metal was discovered in the ruin of an ancient Nordic structure - what appeared to be a temple to the Norse pantheon. With the only other known Nordic temple being the ancient Temple at Uppsala, in Sweden, this find coupled with the Uru metal sparked a curiosity in Donald that set him down a new path: the path of archaeology. He also reunited with Jane Foster at this point, and the two became friends again, though the possibility of rrekindling a romance seemed unlikely at the time.
Armed with knowledge of Norse culture and mythology he'd never known he possessed (and the considerable experience he'd gained the first time around), Donald Blake shot through his second college term in record time, and before he knew it, was off uncovering ancient history across the globe.
The Man Who Found the Captain
During an archaeological expedition in Skraeling Island, Donald - now Professor Blake - discovered a piece of history that, while far more recent than the usual Norse artifacts he had expected to find in the region, would prove to be far more impactful -- the body, frozen solid in ice, of Captain Steven Grant Rogers, looking exactly as it had on the day the bomber went into the North Atlantic. Much to Blake's surprise, Captain America was not dead, but simply trapped in a state of suspended animation. He and his team immediately began work on freeing the Captain from the ice.
While SHIELD confiscated all of Blake's findings, including Steven's body, it did not stop word of the discovery, and who made it, from reaching the world, and before long Donald Blake was a name on everyone's lips...and when Captain America re-awakened and returned to active service as a superhero, a target in the crosshairs of many villains, super and non-super alike.
Jane and Arcade
Scarcely a year after his not-so-secret discovery, Donald Blake found not only himself, but Jane Foster, in the crosshairs of one particularly inventive villain - the eccentric billionaire Arcade, whose method of executing his victims involved abducting them and luring them to his deadly funhouse Murderworld, "Where No One Leaves nhappy (Because No One Leaves Alive!)"