"He remembers. He will return for me here. And when he does... Fire cleanses. And he remembers." —Hunting Log
The Wendigo (Also referred to as the Tall Man or Wendy), is a radically mutated spore carrier in Zion canyon in 2301. It is the result of Joshua Graham's mutation after his death due to his blood infection by a mutant strain the Beauveria mordicana fungus. The mutation turned Graham into a hulking abomination that kills anything unfortunate enough to be trapped in the ruins of Zion.
Before The Fall
Joshua Graham was a New Canaanite and the former legate of Caesar's Legion, but due to losing the battle of Hoover Dam, was made an example of. Caesar had him burned alive and thrown into the grand canyon. Despite his injuries, Graham managed to survive and after a grueling journey, made his way to Zion Valley, where he became acting war-chief to one of it's tribes: the Dead Horses.
Eventually, Graham, the Dead Horses and another tribe in Zion named the Sorrows, were forced to fight for their survival against a tribe of raiders known as the White Legs. Joshua led the Dead Horses while another New Canaanite named Daniel led the Sorrows against the invaders. Sometime into the battle, Zion was visited by the Courier. The Courier helped end the war by convincing the Sorrows to follow Graham in exterminating the White Legs and pushing it's remnants into the Long Dark. (It is truly unknown how the war for Zion ended but given the Sorrows warlike nature in DUST fitting perfectly into how they become after deciding to follow Joshua Graham in one of Honest Heart's endings, this is most likely what happened).
In the following months after the war for Zion, Joshua Graham had warped the minds of the Dead Horses and Sorrows, turning them into tribes as violent as the White Legs and even began to raid like them. He had the Sorrows throw Daniel out of their part of Zion, fearful of his fellow New Canaanite ever reaching out to the Sorrows and reminding them of their peaceful ways.
After The Fall
By 2282, due to possible NCR exploitation of Vault 22, B. mordicana spores were reintroduced into the atmosphere inside Zion canyon, this produced a highly-aggressive form of the fungus and infection rates among Zion occupants increased dramatically. Within a few years of the fungus' reintroduction, Spore carriers and weather conditions had decimated all wildlife previously found in Zion.
The Spore Carriers or "Moss men" as Zion's tribes referred to them, had first resided in Zion in the years immediately after the war. Tribal accounts have that these creatures preyed on the tribal inhabitants of Zion only to be repulsed by a mythological "Father in the cave"; in reality a survivalist who had taken refuge in Zion canyon after the war and who developed an affinity for the tribals who subsequently settled there. In spite of these efforts, Spore-carriers maintained a residual presence in Zion Canyon as recently as 2281 as the spore carrier Fungus B. mordicana was and remains highly communicable.
Sometime afterwards, Daniel was caught by a Sorrows hunting party, and taken to Graham. According to Daniel's notes, Joshua was downright ready to kill him until he learned Daniel had come to warn him of Dead Horse Point being destroyed by those wanting revenge against the now violent Dead Horses. Here it was discovered that Graham had planned to take the tribe out of Zion and effectively abandon the Sorrows. Graham began asking Daniel of what was going on in the Mojave, while Daniel learned of what had been responsible for Zion's devastation.
As of 2283, Daniel was living in Angel Cave while Graham tried mending relationships with his former friend, but the two's different outlooks on how the tribes should behave caused them to clash. When Daniel discovered the tribals were being killed off slowly by Zion's mordicana spores, he tried to have Joshua burn the bodies, but for unknown reasons, the burned man refused, and had the tribals back that decision up.
Becoming the Wendigo
By 2284, Graham was dying, the spores were slowly infecting him, causing his coughing to worsen each day. Graham arranged for Follows-Chalk to take his place as leader of the Dead Horses and Sorrows, presumably to spite Daniel (according to the latter). In December, Joshua finally passed away, with his body being buried in the Aerie, a former ranger outpost in the canyon. Due to fearing Graham being infected, he petitioned to have his body burned, but this was shot down by the tribals.
Within the next five months after his death, tribal hunters began to report sightings of a large and malformed abomination which they referred to as the "Wendigo", a creature from pre-war Algonquin mythology. It was at approximately this time that the introduction of the SM-5 cloud into the Mojave wasteland began to cause adverse weather conditions in the form of a carcinogenic fog that carried the spore carrier virus. Helpless against the spore carriers, the lack of food, the weather and the arrival of the Wendigo, the Tribals fled into the Long Dark, seeking to resettle elsewhere.
Daniel however, chose to remain behind. Over the last few months of seeing the Wendigo, he had become convinced that the Wendigo was in fact Joshua Graham given a dark second life due to the creature's great fear of fire. Daniel locked the Pine Creek tunnel, preventing the tribals from returning under any circumstances, accidentally condemning them to die at the hands of the Divide tribals. Daniel pursued the Wendigo through Zion valley but was eventually slaughtered by the near-immortal creature.
Sometime afterwards, a survivor managed to find his way into Zion, in an attempt to escape the Mojave, this man was former vault dweller Butch Deloria. Butch made his way across Zion, narrowly avoiding the Spore Carriers and the Wendigo, which Butch dismissed as a hallucination and nicknamed "the tall man" and "Wendy". Butch learned from a survivor and former shaman of the Sorrows, White Bird, that the Wendigo was indeed real. The two, realizing their chances of surviving were greater if they stuck together, made their way across Zion, fending off the Wendigo and Zion's other mutants.
Sometime afterwards, Butch and White bird made contact with a woman named Jess and NCR survivors of Wendigo's rampage. Over the next few days Wendigo slowly picked off the group of survivors until only Butch and Jess were left. Despite being injured and possibly infected with the mordicana spores, Butch and Jess managed to make their way to the tunnel leading out of Zion and into the Mojave, narrowly avoiding the Wendigo at the end. Realizing the Wasteland would be brought further into ruin if Zion's horrors ever managed to escape, Butch hid the key to the tunnel's gate in the cave he met White Bird. Trapping the Wendigo and the Spore Carriers inside the ruined park. Since then, the Wendigo has spent it's time roaming the foggy valley it's called home in both lives, hunting for anyone foolish enough to, despite the warnings from it's survivors, enter the park.
The Wendigo has the appearance of a massive and grotesquely mutated spore carrier. Various survivor accounts maintain that it is capable of stretching its extremities in ways that should not be physically possible and that conventional weapons have no appreciable effect on it. It has been noted that the Wendigo appears to be phobic of fire and will flee if ignited though it is questionable how effective this tactic is in practice. Daniel took this trait as proof positive that the creature is, in fact a resurrected Joshua Graham though authorities maintain this as conjecture when they acknowledge the creatures existence at all.
Engagement or contact with the Wendigo under all but the most desperate of circumstances is highly inadvisable. Fire weapons may cause a temporary reprieve from its attacks but all survivor accounts indicate that this creature is not destructible by any known means. This said, the Wendigo is known to avoid waterways and is not active during the early afternoon, so a canny survivor should exploit this knowledge to the end of avoiding contact altogether. It screams as soon as it detects a PC. If you hear it, it is advised to use V.A.T.S. to locate it.
The beast can't enter new locations, so it's a good idea to run to the nearest cave/pre-war closed building if being chased. WARNING: Spore carriers can follow you inside if they detect you.
While it's unconfirmed, Wendigo seems to spawn in certain locations, most notably near some of the pre-war buildings and camp sites.
It seems that the Wendigo disappears when having no place to flee or run to while on fire.