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In the North American instruction manual for Demon's Souls, on page 5 under "Character creation," there are several references to the American television series "The Simpsons."

If one examines the picture in the lower right of the page carefully, they will see that the characters pictured are given names referencing several "Simpsons" characters. These include Bobo, the bear Mr. Burns owned as a child from episode "Rosebud;" Lisa Lionheart, a doll that Lisa attempted to market in the episode "Lisa vs. Malibu Stacy;" and Princess Kashmir, an exotic dancer first appearing in the episode "Homer's Night Out."

There is also a reference to popular American radio broadcaster Barret Hanson, otherwise known as "Dr. Demento."
Contributed by BlazinGaminTaz
Demon's Souls and possibly the rest of the Soul's game series almost did not come to be, as there were problems with building an ideal prototype for the game. According to Hidetaka Miyazaki, "Demon's Souls wasn't doing well [...] The project had problems and the team had been unable to create a compelling prototype. But when I heard it was a fantasy-action role-playing game, I was excited. I figured if I could find a way to take control of the game, I could turn it into anything I wanted. Best of all, if my ideas failed, nobody would care "“ it was already a failure." Miyazaki was eventually assigned to the game after initially working on the Armored Core series and reportedly "changed everything about it."

The initial launch of the game was not favorable, as it only sold 20,000 copies in the first week, far fewer than Sony had hoped. However, good word of the game began to spread rapidly and within a few months, the game had sold over 100,000 copies in Japan, which then prompted a Western release of the game, garnering another 150,000 sales in the first month.
Contributed by G-Haven
An early prototype of the tutorial level is still on the game's disc, and can be accessed via hacking tools. It's essentially the same level as the tutorial level in the final version, but with various differences.
Contributed by G-Haven
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There is an unused model of a child in the game, who is speculated to be the child of the Filthy Vendor from World 5 (Valley Of Defilement). The child has no included animations.
Contributed by G-Haven
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In the final area of World 5 (Valley of Defilement), Maiden Astraea is surround by a number of gold and silver coins. The texture for a group of the coins is actually 20th century French currency, specifically 2 Franc coins.
Contributed by G-Haven
According to a text dump and some unused dialogue from the Crestfallen Warrior in the Nexus, there was a planned "true death" mode. It is unclear if it is just an optional mode akin to "hardcore mode" in other games or if it was previously planned to be a part of the regular game.

The "true death" essentially meant if the player died while in soul form, they would lose more than just the souls they collected beforehand. Due to how it was worded, it was not clear if it meant everything (gear, items, levels, etc) were lost or if the player would keep their obtained gear but lose their levels. The player would also be bound to a "Stone Monolith" after true death, but nothing more referring to the Monolith could be found in the files. It also meant that the player collecting their bloodstains would have not only given them their souls but also their body.

The mode was likely cut due to being too unforgiving, if the intention of it was to be a part of the regular gameplay. In the final version of the game, the bloodstains simply returned the lost souls to the player and they could simply re-obtain their body in a number of different ways.
Contributed by G-Haven
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An unused model of the Hydra from From Software's game Ninja Blade can be found in the game's data. It's registered as an object, as opposed to an enemy. No animation data is associated with it and it was likely meant for testing uses, not to actually be used in the final version of the game.
Contributed by G-Haven
There is unused dialogue of a woman identified as "Hanging Woman." The dialogue suggests that the woman is in pain and misery, and wants the player to cut her down from wherever she is, killing her in the process.

The Hanging Woman is heavily implied to be the daughter of Stockpile Thomas, as in the beginning of the Boletarian Palace (World 1-1) there is a location of two hanging corpses that can be cut down, one holding a female armor set (Old Raggedy Robes) and the other a Jade Hair Ornament. According to the event triggered by Stockpile Thomas, the Jade Hair Ornament belonged to his daughter. It is not known if the Raggedy Robes belonged to her as well, or some other woman, who may have been her mother/Thomas's wife who also did not make it out alive or just another different person.

The event was never finished, as in the final version of the game the two hanging corpses found are actually male and already dead, which implies the two simply picked up the Jade Hair Ornament and Raggedy Robes before being killed.
Contributed by G-Haven
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According to an interview with Game director Hidetaka Miyazaki, there were two level ideas that came close to making it into the final version of the game but did not make it through development. One was a "Library level" and the second the area outside of The Nexus.

It is possible the Library level idea was later reused in From Software's next souls-game, Dark Souls, to create The Duke's Archives.

There is also unused art that is loaded with the environment textures for the Nexus, possibly of the outside area of the Nexus.
Contributed by G-Haven
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The False Idol in world 3-1 (Tower of Latria) holds a book during the fight with her. The pages are textured, which turns out to be of a page from the real-life Book of Kells.
Contributed by G-Haven
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In the game, there are six visible "Archstones" that allow the player to enter levels, but only five of them are useable. The sixth one is visibly destroyed and was originally one for the "land of the giants." According to the story, the Archstone was destroyed intentionally to contain the spread of demons, as it was the first area to be hit by it.

However, there were originally plans to have the area be accessible, with the name of the stage called "The Northern Limit." A large leftover map has been found, with a large number of unused enemies and NPCs;
• Serpent Men, Wolf Warriors, Bear Warriors, Hyena Warriors, Owl Mages, Wounded Wolfs and "Shadow Birds" (A demon-like crow) were set to spawn in the map. The idea of the Serpent Men were reused in the next Soul's game, Dark Souls.
• A very large Yeti-like monster with a large mouth on it's torso.
• "Mimic" Statues called "March planarian" that have two forms; A "passive" statue like one, presumably to disguise itself as a statue, and a "berserk" version, presumably after it has caught the player off guard.
• Two animal-like creatures, a Wounded Wolf and a Crow-like demon called a "Shadow Bird."
• Two unique NPCS. One is a robed man with a monocle that only has idle animations, who is set to load in the second area of the map. The other is a woman which named "Wandering Demon", but may also be called "Filthy One" according to text-dumps. She wears a red dress, has insects crawling on her, wields a serpent staff and was set to spawn in the first and second areas of the map.

It is not known why the area was cut and left unfinished, but it is presumed to be because of time limitations.
Contributed by G-Haven
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If the player equips the Ring of Sincere Prayer, the animation for casting Miracles changes from the default fists held together to the newer both hands open and thrown above the head. This pose reappeared in the next From Software game Dark Souls as the ever popular "Praise the Sun!" gesture.
The pose itself was almost never included in Demon's Souls, as according to director Hidetaka Miyazaki "When I presented the game to the rest of the company I showed them that pose and one of the higher ups told me it just wasn't cool enough. Of course I told him I'd get rid of it but I secretly kept it in the game."
Contributed by G-Haven
The Old Monk, the boss of world 3-3, is arguably one of the most unique bosses in video game history. While at its base the fight is merely a Black Phantom enemy NPC summoned by the Monk himself, when Online Play is activated the Black Phantom can actually be another player. Any players with summon signs in the Tower of Latria can be summoned to play the role of the Monk's Black Phantom. The Black Phantom players retain all their equipment and abilities, as well as gaining some innate magic attacks by the Monk, giving the boss fight incredibly diverse possibilities.
Contributed by TomGuycott
Though difficult, it is possible to defeat the Vanguard at the end of the tutorial stage. Doing so will lead the player to a room within the Dragon God's lair in the Stonefang Mine, where several Grasses and Stones lay as a reward. However, after going as far as they can a cutscene ensues where the Dragon God kills the player in one blow, setting in motion the events that would have happened by dying at the Vanguard's hands instead.
Contributed by TomGuycott
Throughout the game, mention is made of several heroes of Boletaria who were devoured by the fog along with King Allant. When Allant returned he, as well as all his men, were said to actually be demons taking their forms. In World 1-4, the Black Phantoms of the original heroes block the path.

Alfred, the Knight of the Tower, became the Tower Knight in 1-2. Curiously, this phantom also wears the armor of the Twin Fangs, Vallarfax and Biorr, the corpse of the latter supposedly in Mildred's excecution grounds in 1-1.

Metas, the Knight of the Lance, became the Penetrator in 1-3. Curiously, both the demon and Black Phantom use an incredibly long sword, not a lance.

Long Bow Oolan is left out, though it is said Oolan led a legion of archers, a characteristic similar to the legion-like qualities of the boss Phallanx in 1-1. This is debatable, though, seeing as Phallanx's legion actually uses spears and shields, and its soul forms the Scraping Spear that Alfred's Black Phantom uses.
Contributed by TomGuycott