The Gate of Truth is a central nexus of all knowledge in the universe, or something close to that. Tends to be difficult, if not nearly impossible to reach. If the answer is Forty-Two, then chances are, the question is somewhere in there. It may or may not be sentient, and may or may not be actively collecting knowledge, but what's important is that it already has a lot of it in stock. If it's known at all to people, those who know about it are very likely to be an elite of some sort, and will more likely than not be killing each other just to get there.
God, or something like that might be guarding it to protect all the secrets within, if they are not the same being as the gate itself. Else, there's probably a toll-keeper who requests a small fee before you can take a peek at its vast repository. The point is, the knowledge is collected, and a block has been installed that keeps everyone from just wandering up and partaking of it.
Whether or not the knowledge gained was worth the cost depends upon on which end of the Sliding Scale of Idealism Versus Cynicism the story lies.
Note that the Gate of Truth need not necessarily be a literal gate. It could be a tree, a scary door, or even the corner of Rhodes and Fourth Street. "All knowledge" need not be a literal description, either. Simply a lot will do.
- The Gate of Truth from Fullmetal Alchemist. Truth, a white, featureless, jerkass godlike being whose physique matches that of the person entering his realm, is the gatekeeper. You pay with a part of yourself, which Truth then wears until you reclaim it, assuming you ever do; in exchange, you gain the ability to perform alchemy without a transmutation circle. At the end, it is revealed that it is possible to pay with your Gate instead of your body parts; however, this renders the alchemist unable to perform alchemy ever again, and unless he somehow is tied to another person's gate, will strand him in Truth's realm until he dies, and that's if you die. This places FMA clearly on the idealist side of things; the knowledge that Edward gained from the Truth didn't really help him all that much; more than that, he started to believe that the knowledge he gained could help him overcome any obstacle, and thus he became Truth's puppet. In the end, although he loses his ability to perform alchemy, as long as he has his friends, he's happy.
- Works slightly differently in the 2003 anime version. There is no gatekeeper; rather, the Gate itself is implied to be sentient. Also, the Gate is, in fact, a portal between the world of the series and our own world, and thus the source of the soul energy that powers Alchemy. It is inhabited by weird black imps that do the body part-claiming in Truth's stead — and for a short period of time, the homunculus Wrath until Ed provides him with a means of escape. It's implied the imps are actually the souls of those who die in our world. Correspondingly, the design is quite different, lacking the Tree of Life on the door and looking a lot like Auguste Rodin's sculpture The Gates of Hell.
- Mnemosyne has Yggdrasil actively trying to become this by casting time spores on the world and using them to gather data.
- The Galactic Leyline from Outlaw Star is mainly one of these, though it also grants wishes. If you can reach its center in one piece.
- The quantum computer from Battle Angel Alita: Last Order. It uses the collected brains (the collective unconscious of humanity) as a means to predict the future. Zalem keeps it a heavily guarded secret from everyone.
- In the Nasuverse, mages know the compendium of all knowledge in the universe by several names: the Root, the Swirl, the Origin, Akasha, etc. The ultimate goal of magic is said to be to find a way to access it. They are willing to go some extremes if they think there's a chance of achieving this goal. The Holy Grail War in Fate/stay night is just one attempt at this, and The Garden of Sinners goes into a bit more detail on just what the Root is and why the mages seek it so desperately.
- The Rose Gate in Revolutionary Girl Utena qualifies as such, this gate which appears in the last episode presented as Akio's end goal with the Dueling Arena set ups: groom a champion that unlike him, will be noble enough to bear the Prince's sword, dispose of them and attempt to open the door with it himself, behind that door lies "the power to revolutionize the world" Or in his own words:
Akio/The Prince: Do you see that Rose Gate? Through that door lies eternity, that which shines, the power of miracles... with that power, anything is possible. It's too much for you, you're just a girl aren't you? And so, you will not have that power,if you had it you could do anything, you could free that girl from her destiny. But how that power shall be used... is my decision.
- The Seven Gates in The Promised Neverland are what someone has to pass through if they want to make a deal with the nameless demon god.
- In Planetary there's a supercomputer composed of thousands of souls under the streets of Hong Kong, it chooses a "champion" in the form of a cop killed by their partner as betrayal.
- The Source in The DCU is close to this trope. The main discrepancy is that it's not a gate, it's a wall composed primarily of the paralyzed/perhaps dead bodies of former god-like beings who tried to breach it.
- In The Odyssey, there are two gates through which all dreams pass, one of horn and one of ivory. Dreams that pass through the horn gate are true (that is, they come to pass in Real Life), whereas dreams that go through the ivory gate are deceptive and false.
- In Neil Gaiman's Neverwhere, the angel Islington guards, among other things, the door to Heaven, hidden in the deepest part of the London Underground.
- From the Cthulhu Mythos, Yog-Sothoth is the Gate. Yog-Sothoth is the Key. Yog-Sothoth is the All-In-One and the One-In-All. Past and present are one in Yog-Sothoth. He alone knows where the Great Old Ones walked the Earth's fields, and He alone knows where they shall do so again and why none can behold Them when They walk.
- "Deep Thought" from The Hitchhiker's Guide to the Galaxy was designed to be one. When asked to devise the Answer to the Ultimate Question of Life, the Universe, and Everything, it comes up with the answer of "42". When scrutinized for how such a ridiculous answer could have been the solution, after over the course of 7 and half million years, it simply stated that IS the answer, but was never told the actual question. A far greated computer would be needed, it would take far longer to solve, and organic life would be part of its computational matrix. The Earth, which was the real Gate of Truth capable of giving any answer. The solution had just been devised by a woman named Fenchurch who was rushing to tell someone, anyone, when the planet was destroyed so that a hyperspace bypass could be opened in Earth's orbit. When Arthur Dent ended up in a parallel quantum timeline where Earth hadn't been destroyed, Fenchurch instead suffered an inexplicable medical episode and passed out, forgetting what she had figured out. By this point, Arthur was sick of the whole Question nonsense and gave up on it.
- The vignette "A Man Before the Law" in the Franz Kafka novel The Trial is a subversion, in that the man trying to enter the gate that leads to the law itself never gets to enter despite spending an entire lifetime trying to learn the required price.
- Many fictional uses of the Real Life Library of Alexandria, which was lost or destroyed depending on which story you believe, have it become one of these.
- In The Dresden Files, The Archive or "Ivy" is a living example, as she knows everything that has ever been written, and is updated in realtime, to the point that Harry sends her a note using this. It's a bloodline ability, passed from mother to daughter.
- The Cthaeh in The Kingkiller Chronicle is an omniscient being that was sealed in a tree in the Fae realm. Unfortunately, it's actively malevolent, so it carefully picks its words in order to create the worst outcome possible.
- In Doctor Who
- Rose Tyler looked into the Heart of the TARDIS and it poured all the information in the universe into her head, turning her into a God-like being called "Bad Wolf".
- Also later in the series, the Doctor describes all the time lords looking into hole of space and time as children and seeing different things, some seeing all the secrets of the universe.
- The Artifact in season's 1 & 2 of Eureka acts in this capacity. It serves as a connection to the "Akashic Field", a repository of all knowledge in the universe. Forming a connection to The Artifact gives great power, including near instant regeneration telepathy and telekinesis, but is more than a normal human mind can handle. Only 2 characters survived initial contact with it, both displaying abnormal mental function prior to contact, with one suffering a Disney Death at the end of the episode and the other being freed from it before the power could grow too great.
- The Black Library in Warhammer 40,000 contains every piece of information about Chaos ever collected. It exists in an unknown location deep in the Eldar's Webway, is guarded by the secretive, dangerous Harlequin cult of Eldar warriors, and only a handful of humans (most notably the Emperor) have ever reached it. Considering that much of the information is Things Man Was Not Meant to Know, the security is necessary.
- A.B.A's instant kill attack in Guilty Gear causes one of these, inscribed with the Kabbalic Tree of Life to appear, open, and suck the enemy in on a successful hit. The door slams shut, and is then locked shut by her giant monstrous key. "Destroyed!" intones the announcer.
- The Elder Scrolls:
- Apocrypha is the Oblivion realm of Hermaeus Mora, the Daedric Prince of Knowledge. It is an infinite library containing all the world's knowledge. You get a chance to visit it yourself in Skyrim's Dragonborn DLC.
- The Elder Scrolls themselves are a portable version, being both Tomes of Eldritch Lore and of Prophecy and Fate. They allow one to glimpse the very fabric of reality and decipher knowledge. They come with some rather nasty side effects, however, including blindness and madness.
- In Kingdom Hearts, the titular Kingdom Hearts - the Heart of All Worlds - is hidden behind the Door to Darkness, a massive doorway found deep within the End of the World. Beyond that lay the Realm of Darkness where the Heartless hail from.
- The final titan in Titan Souls is more than implied to be the guardian of the truth itself.
- There are several doors in Cultist Simulator a character can pass through and enter Mansus. Some of them bestow knowledge. Some of them require knowledge to pass before you can gain more knowledge from the other side. Some of them can only be accessed in dreams. The game loves this trope.
- The Seer from Kid Radd. Everyone who takes his advice ends up dying in some contrived way, usually because they took his advice.
- The Valley of the Oracle in The Order of the Stick is a test of prowess and wit for those seeking the Truth, and if they succeed, they can get a True answer to any question. But they can't take the whole truth with them; on leaving, the Memory Charm placed on Sunken Valley wipes their memories of everything except their question and the answer.
- Although Roy accidentally discovers a way to work around the Memory Charm: as a ghost, he annoys the Oracle enough that he banishes Roy (using a magic item, the Oracle has no actual Divine magic), not realizing that banishing Roy bypasses the Memory Charm.
- SCP Foundation: SCP-028, aka. 'Knowledge', is a spherical area inside an otherwise mundane abandoned storage yard that, when entered by a human, provides that person with complete and precise knowledge on one specific but random subject. The same person can enter SCP-028 multiple times, but trying to enter more than twice will start causing increasingly debilitating physical effects including migraines and dizziness. The knowledge obtained by various subjects ranges from the location of a lost set of car keys, the full contents of the 1998 New York phone directory, how to rebuild an internal combustion engine to run on human bloodnote without adding or removing parts, how to recycle carbon dioxide inside one's lungs, and... well, this:
Incident Log: Subject D-1182 exposed to SCP-028. Subject began to cry and went into fetal position, showing signs of high distress. Unresponsive to questioning and outside stimulus for several days. Lapsed into catatonia shortly after stating that this is not life. Subject passed into a coma and died shortly thereafter. C.O.D. was attributed to shock.
- Avatar: The Last Airbender:
- Wan Shi Tong's Spirit Library is this, complete with scary spirit guardian. There's a reason his title is "He Who Knows Ten Thousand Things". He also has fox servants that regularly bring back more knowledge.
- The spirit world itself; Aang has to brave various monstrous terrors so that he can ask questions of his previous incarnations.
- An episode of Justice League features the library of Hades himself, located in you-know-where.
- The Matrix of Leadership from Transformers: Generation 1. When opened, it destroys an intergalactic Hate Plague by unleashing its accumulated wisdom. In this version at least (it's a recurring MacGuffin in the franchise but its exact nature is not always 100% the same.) it contains the wisdom of every previous Autobot leader and through it Optimus could contact avatars of his predecessors (and Rodimus could contact them, including Optimus, as well.) With it emptied to end the Hate Plague, it must start again from zero.
- Among the many Shen Gong Wu in Xiaolin Showdown is the Fountain of Hui, which bestows any and all information upon the user. By itself, it only provides random knowledge (and a really bad migraine), but when paired with the Eagle Scope, one can sort through all the junk and find whatever specific bit of info they're looking for.
- Star Wars Rebels reveals that when the Sith and Jedi holocrons are merged, any question the seekers ask can be answered. There's three major reasons why this hasn't become a Story-Breaker Power: one, only a few people in the entire galaxy are aware of this, two, it requires a Sith and Jedi to cooperate, an incredibly rare occurrence, and three, obtaining those secrets will break your mind. Maul and Ezra manage it, with Maul seeking hope and Ezra seeking the key to destroying the Sith, but Ezra breaks the connection before it gives him the answer. Maul gets the answer he wants and loses it (although he wasn't exactly stable beforehand), while Ezra gets only scattered images telling him where to look and a major headache.