The Golden Throne
The Golden Throne is the giant, elaborate life-support chair and psychic amplifier where the God-Emperor of Mankind's decaying form currently resides, deep within the bowels of the Imperial Palace, under the armed guard of the Adeptus Custodes.
Origins and the Original Purpose of the Throne
The core of the thing that would eventually become the Golden Throne was found by Big Daddy E during the Unification Wars waaaay back during the Age of Strife, wherein he found a curious piece of tech buried in some ruins out in the middle of nowhere in what was once known as Asia, as well as it's testing bed tech called Dark Glass. He immediately went out trying to get it repaired and refurbished, and began building around the core deep within the dungeons of what eventually became the Imperial Palace, adding more and more to it until it was the big enormous gold-plated throne stuck to more cables than there are stars in the sky that would satisfy the big man himself. It's purpose? To rid man of use of the Warp for good, and to open the Webway up to the Imperium of Man.
As you're no doubt aware, the Warp is where Daemons and more importantly, the power of the Psyker comes from and the Emperor HATED that. He wanted Mankind to be completely and utterly free from the evils of Chaos, and the tempting power of the Warp...but the big issue he faced was that the Warp is also where most of his massive spaceships and his armies had to go in order to get anywhere. The solution? Use it's massive psychic amplication abilities to open a hole into the Webway, and use it to unite humanity's worlds and thrive in a Galaxy seeming designed to shred it to little bitty pieces. If he could do that, then he and whoever else in the Mechanicus could isolate the gene in the human genome that allows connection to the Warp, isolate it, then either gene therapy it out of the species or blam those who resisted, and finally get Humanity away from the powers of the Dark Gods. Sure, there was a minor problem in that it needed a really, like...really powerful psyker to actually make that feasible, but hey! He had at least a couple of Primarchs kicking around who could probably fill that role! It's all good!
So what happened to that?
In short, Horus Happened, and Magnus fucked up at the worst possible time. (What!? Magnus did nothing wrong!)
Not long after the Emprah put Horus in charge of going out and killing in his name, he went back to Terra to figure out the more administrative aspects of the Imperium of Man. This included setting the final groundwork for mankind’s access to the Elfdar's portal-realm - the Webway. The project was top-secret, even the Primarchs didn't know of its existence. This along with the creation of the Council of Terra, kinda pissed-off his sons, at which point a couple of them decided the Emprah wasn't their REAL dad and threw a big hissy fit. Malcador, the Emperor's right-hand, and Rogal Dorn, Primarch of the Imperial Fists, decided to roll up on the Palace and see what the fuck was going on, wherein He received them and FINALLY spilled the beans on his ‘master plan’. Elsewhere, Magnus the Red discovered the Chaos Gods were trying their good goddamnedest to get that Horus Heresy thing going full tilt. In desperation he made use of a sorcerous ritual to warn Empy shit was going down...and that shattered the psychic shields around the Imperial Webway extension, allowing all sorts of Warp Nasties to enter with the possibility of invading Terra itself (not a good thing). This prompted the Custodes and Sisters of Silence to engage in a vicious five year battle to defend mankind’s claims in the Webway. They did a good job, but eventually had to retreat due to the grinding attrition of the endless demon-tide. The Emperor used his considerable psychic powers to manifest a blinding light that repelled the daemons, buying time for the survivors to make their way back to the Materium. He then forced the gate shut, closing humanity from the Webway, and the daemons from Terra.
Problem was, the only way to keep the damn thing closed was to be a powerful Primarch or better in terms of psychic power to keep the daemons on the right side of that gate. So from then on, the throne went from a Psychic beacon and possible savior of mankind, to the Emperor's prison, ensuring that he'd have to sit there and keep Daemons out of his yard while Horus went about fucking things up until Malcador came back and basically begged him to get up and go deal with that, and he took his place on the throne for several days and was essentially boiled alive by psychic power after being on it...and then the Emperor came back to the Imperial Dungeon, barely clinging to life. The Techpriests and Dorn pulled him off of it, and the Emperor was returned to the throne, where he gave a quick speech, and quietly took his place back on the gold-plated toilet, and he's been there ever since for the last several thousand years.
Post-Heresy: The Imperium's Golden Life Support System
Nowadays, the Throne maintains a lot of things vital to the Imperium's continued survival: The Astronomicon, The psychic lock on the abandoned section of the Imperial Webway, and prolonging the life of the Emperor of Mankind, now only a half-dead corpse. The throne's rapid changes and modifications have caused a lot of unintended consequences: Namely the need for a constant sacrifice of Psykers, which facilitated the creation of the Black Ships. And even then, the Techpriests that maintained the throne found that the usual sacrifice of psykers wasn't cutting it in 36M, and they shot the number up by four times the usual amount, forcing the ships to go looking for even more psykers than they normally did; making a dangerous job considerably even more dangerous.
Further, after one of many of Failbaddon's Black Crusades, it was discovered that the Throne had some...problems. Namely, a lot of the technology that kept the thing working were starting to fail, and the Techpriests and Mechanicus at large can't fix any of them which is...bad. As for how bad? It's got several Mechanicus Cults going around trying to find ways to fix it or come up with a contingency for the eventuality of if/when this thing goes out. As for what happens? Well...Nobody's really sure. It's discussed at length over on Big E's page, but a commonality is obvious: Something BAD will happen. And in the Indomitus era, where the Custodes have decided that they're actually gonna go out and do shit for once, they decided that the best solution is to just fix the damn thing, and are looking towards going to specific lost Forge Worlds in order to find what they need. As for whether or not they succeed? That's Advancing the Storyline, my man. We'll probably find out in 10th edition.
The Vaults of Terra series gave a lot more information about the Throne, as well as its current problems. Firstly, the Throne is huge. As in the whole machine is larger than most hives. It's also confirmed that the "original" throne was made with an amalgamation of technology from multiple sources, including from xenos races. The Mechanicum have also known about its impending failure for quite a while, and have tried everything in order to fix it, hence why the current fabricator General became part of the conspiracy to get the Dark Eldar to help fix it.