(The following story is told among Doomguard lore keepers, illustrating the ultimate failure of every "everlasting" social system, and the destructive folly of both primes and planars.)
A wise and mighty man lived somewhere on the Great Ring. Learning the secret of astral travel, he sealed himself inside his home with unbreakable barriers, and sent his projected spirit through the Silver Void, spiraling down toward the Prime Material.
Emerging from the Astral, he saw a primitive world bristling with life and rich with potential. He said to himself: "This land shall be my dominion, and my heirs' for ages to come!" Thus sworn, he sought out the simple-minded human beings who lived there, and awed them with his presence, power, and wisdom. He became their lord and ruled for countless years, teaching them all they knew... but not all he knew.
The Emperor's subjects learned agriculture and architecture, magic and warfare, music and poetry ... but they never shared the Emperor's planar lore, for it would not do to have them learn of other worlds beyond his control. The Emperor explained that his house is in the heavens, and that he comes to grace them with his divine presence by descending a "heavenly rope".
The Emperor had many sons, and when he grew weary of false immortality and his life on the Prime, he took the eldest to the sealed citadel on the planes, taught him the secrets of astral projection, and left to live out the rest of his days away from his former empire. The nameless prime world mourned the old and celebrated the new Emperor, who was almost as wise and caring as his father was.
The succession process was repeated many times over the centuries, and with every generation, the ageless Emperors lost a small part of their ancestor's lore and spirit, while their people learned a little more on their own and grew wiser, slowly beginning to question the authority of their monarchs.
By the time of the seventh son of the original Emperor, the people began to ask difficult questions. Many a "why" was met with a "because We say so", as wise and noble men began to lose their heads. To pacify the people and strengthen the dynasty, the Emperor of this age established a customary duel during which he would meet in combat the finest warrior or mage amongst his noble subjects. Should the noble win, his family would be the new royalty. Should the Emperor win the family would forever lose their status, falling into the lowest of the eight castes. Needless to say, each Emperor easily won every challenge astrally projected from the safety of his planar home.
During the rule of the seventh son of the seventh son of the original Emperor, a particularly cunning vizier found mention of a "heavenly rope" among old stories, and came to understand that each Emperor's power is somehow transferred from the Heavens over this connection. Now, this vizier was not an expert warrior or a mighty mage, only a nobleman with a skill in manipulating people. He made it public knowledge that he would be this generation's challenger against the Emperor.
As the news circulated throughout the Crystal City, many wondered if the vizier had finally lost his senses. Would he, with little skill in either the arcane arts or warcraft, submit his old and proud family to the ultimate dishonor? Would he challenge the invincible monarch with no hope of success? However, many remembered what a cunning and devious man the vizier was, and that the great confidence with which he spoke of the upcoming challenge simply could not be unfounded. When questioned about his secret plan, the vizier smiled knowingly and denied everything.
Finally, the rumors reached the ears of the Emperor, and despite his supreme dominance, doubt crept inside his soul. He send his best spies to dig up the vizier's darks. The spies soon reported that the vizier had, in what he thought were secure chambers, shared the truth with his closest friends and family. Apparently, a prophecy had assured him that he would never be defeated until he faces a foe wearing a dead fox over one shoulder, a dead mouse over the other, a bag of ashes around his mount's neck, and wielding a silver sword. "No one in his right mind would wear such ridicules attire in a duel, and we are all familiar with the softness of silver weapons. I can not be defeated in the duel, no matter what powers the Emperor commands."
Hearing this news, the Emperor laughed with relief. He knew any such prophecy must be pure screed, but he decided he would still equip himself with a pair of animal corpses, tie a bag of ash around his destrier's neck, and wield a magical silver sword (recently retrieved from his ancestor's astral Citadel). And all this for no other reasons other than to see the look on the vizier's face when his doom approaches.
The day of the duel finally came, and the people gathered to see the highly anticipated event. As the Emperor advanced, the courtiers marveled at his unorthodox apparel, and the vizier fell to his knees in silence, weapons dropped, head bowed. The Emperor laughed bitterly and proclaimed, as his predecessors did many times before, the folly of willingly challenging the royal family. As he spurred his mount and charged at the kneeling man, the bag of ash ripped open and blinded both the mount and the rider. The Emperor whirled about in panic, calling upon the two celestial spirit-advisors that always floated by his head to protect him from harm, but the spirits had fled disgusted by the bloody corpses that the Emperor had draped over his shoulders. In his blind rage, he swung his sword around aimlessly, expecting the challenger to strike against him at any moment... and soon enough, the silver blade sliced through the astral cord connecting the Emperor with his true body in the planar Citadel, ending both his life and his projected presence on the Prime.
The vizier stood up, brushed the dust off his robe, and proclaimed himself the new Emperor. Now, it would be nice to state that he led the Empire into a new era of revelation and progress, but it would also be untrue. Most of his reign he spent searching through the old Emperor's writings, looking for the secret of the "heavenly rope" that would make him immortal and invincible, but of course, he never found it. He died at the age of fifty, shortsighted and bitter. He left no heir, and the ancient empire soon dissolved in succession squabbles and old feuds.
The people spoke of the golden age of celestial emperors and sighed. "Nothing lasts forever," they would say "Not any more."
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Author's note: This story was inspired by a Tibetan legend about seven generations of kings climbing down a rope from the heavens, defeated by a tricksy minister pretty much as described above. In case you're interested, a Google search for "heavenly rope" and "Tibet" may find this chant.
Neat. So the prophecy was actually meant for the Emperor and not the vizier?
This is the only part I would reword. It doesn't quite convey what you intended (that they learned everything from him):