The Tall Man is the name given to one of the primary antagonists of the Chzo Mythos horror adventure game series. He mostly resembles a human, albeit an inhumanly stretched-out and bandaged one, but his actions are anything but. Little is known about his motives or powers for most of the series, as he seems to materialize wherever is most inconvenient for the protagonist, always wielding an axe which he uses to brutally murder anyone in his path. The Tall Man is the second villain to be introduced in the Mythos, after John Defoe -- for Trilby, encountering the Tall Man during his visitation to Clanbronwyn Island in the third game is so viscerally horrifying and unlike the previous terror of Defoe, that he later feels pity for Defoe instead of fear. This reaction tragically leads into the events of 7 Days a Skeptic.
Although his actual name is given during the events of Trilby's Notes, he is still known as "The Tall Man" for most of the series. In the Books of Chzo, "religious" texts penned by the followers of Chzo, he is known as The Prince, and punctuates nearly every chapter by teaching a non-believer the name of the King.
The designer and writer of the Chzo Mythos, Ben "Yahtzee" Croshaw, says in the commentary on the premium edition of Trilby's Notes that the Tall Man was designed from the ground-up to be scarier and more unknowable than John Defoe, so that he could introduce the more mystical, Lovecraftian aspects of the series such as the Order of Blessed Agonies and Chzo himself. His goal was to create a character who could be a wildcard, and become wholly terrifying for the remainder of the series, since the players already knew too much of the backstory to Defoe.
Defoe resembles Jason Voorhees and Chzo resembles Cthulhu, two horror icons who stand on complete opposite ends of the horror spectrum -- the former being violent, visceral action horror, and the latter being eldritch, fear of the unknown horror. The Tall Man expertly combines these two aspects: almost-but-not-quite-human, slow-moving but deliberately violent at all times, totally unknown and unstoppable. He does, in fact, bear many similarities to the Slender Man, who was invented just a few years later.
The other two villains of the Chzo Mythos have their place, but the one who leaves the biggest impact is undoubtedly the Tall Man. By the end of the series, John Defoe is more of a tragic figure than a truly scary antagonist, and Chzo spends the majority of the games as a chessmaster, making only brief appearances until the very end of 6 Days a Sacrifice. Even though The Tall Man is only visible in half the series, the impact he leaves was enough to keep me awake for days the first time I played through the Mythos. He's terrifying in a way that Slender Man could only ever dream of being.
Full History (with spoilers)
Since part of the horror in the Chzo Mythos is fear of the unknown, it is impossible to give a full history of the Tall Man without spoiling late-game events and ruining some of the games' charms. Stop reading now if you have not played the games and do not wish to be spoiled.
Despite his alien appearance, The Tall Man did actually begin his existence as a human. He was a Celtic druid in the 1st Century, then named Cabadath. During those times, the barrier between the Realm of Technology (our world) and Realm of Magick (Chzo's world) was weaker than in modern times, and advanced druids like he were capable of performing basic feats of sorcery. Sometime during Cabadath's human life, an invasion by Roman forces threatened to destroy his settlement. Cabadath decided to draw upon the power of Magick to perform a ritual which would bring the ancient pain elemental Chzo into the Realm of Technology. However, he underestimated the power of Chzo, who instead decided to pull Cabadath into his own realm, where he was stretched-out and tortured, forced to serve Chzo for all eternity as the Tall Man.
Chzo placed the soul of the Tall Man in the wood of the tree from which he had performed the ritual. With this deed done, the Tall Man became the only being of Magick who could materialize in the Realm of Technology, though he could only do so on July 28, the day he initially summoned Chzo. A long series of events spanning centuries is told through flashbacks in Trilby's Notes, which explain the long, complicated history of The Order of Blessed Agonies -- a cult formed by Jack Frehorn to worship Chzo, who believe that perfection may only be achieved by undergoing purification through suffering.
As Chzo is termed a King by his followers, they also, by association, choose to term Cabadath as the Prince. In the Book of the Prince (volume one in the Books of Chzo), the initial human form of the Prince is called The Arrogant Man due to his arrogance in assuming that he could control Chzo. His enslavement is deemed a gift.
These flashbacks also tell how the wood containing the Tall Man's soul eventually wound up in a carved idol owned by Sir Roderick Defoe, father of John Defoe. One horrible night, Roderick used that idol to beat his son to death in a fit of drunken rage. The Tall Man's soul entered the body of John Defoe on that particular night, which just so happened to be another July 28. This turned John Defoe into a wraith, a wretched abomination formed of Magick and Technology who was doomed to forever serve Chzo in the Realm of Technology.
At the end of 6 Days a Sacrifice, the final game in the series, it is revealed that the entire reason Chzo created the Prince in the first place was so that he could create John Defoe. Chzo is a being of Magick whose powers are weakened significantly in the Realm of Technology -- he wanted to create a being capable of transcending the barrier between the two realms, ensuring him a fresh supply of pain even when his own realm ran dry. In order for the Prince to be this being, however, he would have had to have undergone purification through suffering to become a perfect slave. The Prince, being formerly known as the Arrogant Man, was always "sinful" in the eyes of Chzo, unworthy to be the true Prince. John Defoe had the additional problem of being created by the Prince, and therefore being reliant on the Prince's continued existence. However, because John Defoe was a being created by both Magick and Technology, he was essential for creating a new Bridge between the two realms.
Ultimately, Jack Frehorn's prophecies, given to him by Chzo centuries before, lead the Order of Blessed Agonies to lure the Tall Man and John Defoe to one central location in 2189. This location is where Chzo formed the Bridge which allowed him to directly affect the Realm of Technology for the first time in millenia. His followers believed that they were fulfilling these prophecies so that Chzo could enter our world and purify all the non-believers, but in actuality Chzo had no desire to enter our world -- he was a being of Magick and only sought to use the Bridge to bring across one person who had undergone purification, who would become the New Prince and destroy Cabadath once and for all.
In the end, the story of the Tall Man teaches one lesson: don't mess with an all-powerful eldritch horror, because he will launch a scheme that takes over 3000 years and millions of deaths just to get back at you.