Much is known about the rule of Moctezuma II, the great 16th-century supreme military commander and "divine leader" who inspired worldwide awe with his battle-hardiness and wondrous ability to mediate with the Gods.

Historians have marvelled at his creative achievements over the largest empire in Meso-America, feared for its military might and bloodthirsty human sacrifices. The tale of his downfall, on the other hand, is a cautionary example of the dangers of trust: the Aztec ruler met his end at the hands of the Spanish conquistadors who arrived on his shores and apparently beguiled him into a friendship that ended in incarceration and death.

Yet after centuries of study and archeological discovery, Moctezuma, the man, remains virtually unreachable to historians.

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Almost nothing has been gleaned about the personality of the last great elected Aztec ruler, who leaves the loyalties of Mexicans utterly divided to this day because he is believed to have colluded with the incoming Spanish colonial power against his own people. Now, though, a new blockbuster exhibition at the British Museum will attempt to solve some of the mysteries surrounding a myth that has long been impenetrable.

"Moctezuma: Aztec Ruler" is the fourth and last in the British Museum's series of big shows about emperors. It will present a revisionist view of the leader that looks set to reveal the monarch in more intimate detail than has ever been possible before. That progress is the result of new evidence emanating from an archeological study currently being undertaken in Moctezuma's palace, in what is now Mexico City.

The show's curator, Colin McEwan, admitted that personal details about Moctezuma are so scarce that one academic thought the exhibition, which opens on 24 September, would be impossible.

Even first-hand descriptions of his life are full of contradictions, which means that the style in which he ruled, to say nothing of the character of the man, remain elusive.

But there are some details that can be pinned down. We know that Moctezuma II, or Montezuma as he is sometimes called, was the last elected Aztec emperor and ruled over an empire that stretched from the shores of the Pacific to the Gulf of Mexico. As a battle-hardened general, he was appointed supreme military commander before being elected as "ruling lord" in 1502, when he built a new palace in his capital Tenochtitlan (modern day Mexico City).

While we will never have the benefit of a full character study, we can surely glean something of the emperor's personality from his name, which means "he who frowns like a lord".

By all accounts, this was a lord with good reason to frown: Moctezuma wrestled with a great number of woes during his reign. Although he was seen as a cunning and fearsome legislator, heavy centralised taxation provoked resentment in outlying areas. And that headache paled in comparison with the problem of how to deal with the arrival of the Spanish conquistador Herná*Cortés, who landed at Veracruz with just a few hundred men in 1519.

When Cortés attempted to march on to Tenochtitlán, he was well received by Moctezuma, but that kindness was not returned, and eventually the emperor was taken prisoner. Although he was eventually restored to the throne, he was never again more than a vassal of Spain: the long-accepted version of events is that dissident groups among his people rebelled and stoned him to death. But the truth, the British Museum suggests, is rather different. In fact, the new exhibition claims, he was ruthlessly murdered by the Spanish when he was no longer of use.

Curators point to the evidence gathered in the new show, which will display together for the first time two 16th-century manuscripts brought in from Mexico and Glasgow University. The documents feature small figures among a wealth of detailed illustrations of the first encounters between Aztecs and Spaniards which have have only recently caught scholars' attention. In the images, both manuscripts show Moctezuma shackled or with a rope around his neck.

Descriptions of the emperor's death have documented the grief felt among the colonial force. The account of Bernal Díaz del Castillo's "True History of the Conquest of New Spain" portrays a noble leader who won the hearts of his captors.

He wrote: "Cortes and all of us captains and soldiers wept for him, and there was no one among us that knew him and had dealings with him who did not mourn him as if he were our father, which was not surprising, since he was so good.

"It was stated that he had reigned for 17 years, and was the best king they ever had in Mexico, and that he had personally triumphed in three wars against countries he had subjugated. I have spoken of the sorrow we all felt when we saw that Montezuma was dead."

Díaz del Castillo went on to provide a detailed description of a debonair leader who cared about his personal appearance, kept "many mistresses", and had a grand total of 19 children – 11 sons and 8 daughters.

"The Great Montezuma," he wrote, "was about 40 years old, of good height, well proportioned, spare and slight, and not very dark, though of the usual Indian complexion. He did not wear his hair long but just over his ears, and he had a short black beard, well-shaped and thin.

"His face was rather long and cheerful, he had fine eyes, and in his appearance and manner could express geniality or, when necessary, a serious composure. He was neat and clean, and took a bath every afternoon.

"He had many women as his mistresses, the daughters of chieftains, but two legitimate wives... The clothes he wore one day he did not wear again until three or four days later. He had a guard of two hundred chieftains lodged in rooms beside his own, only some of whom were permitted to speak to him."

But not every observer agreed with that essentially benign image. Bernardino de Sahagún, a Franciscan missionary to the Aztec people, for instance, portrayed Moctezuma as a weak-willed, superstitious and indulgent ruler.

Some historians, including James Lockhart, explain that disparity by suggesting that when the Aztecs were searching for a scapegoat for their defeat, Moctezuma was unfortunate enough to fit that role. Claims that he was killed by his people could be seen to confound that myth.

Whatever the truth about Moctezuma's demise, one thing is certain: the arrival of the Spanish represented the collapse of the naive world order and the imposition of a new civilisation that ultimately gave birth to modern Mexico.

Unveiling details of the exhibition, British Museum director Neil MacGregor said the story of Moctezuma presented "perhaps one of the most fascinating examples of implosion of power and the clash of civilisations".

Moctezuma's reputation is still contentious in Mexico, according to MacGregor. "There has never been an exhibition on this man, a great emperor of an extremely sophisticated empire," he said.

The exhibition will bring together spectacular loans from Europe and Mexico, including 132 objects, some of which were recently excavated from remains of the Aztec city. Next year it will coincide with the anniversaries of the independence of Mexico in 1810 and of the Mexican revolution of 1910.

If the picture of Moctezuma is still a little hazy, there remains hope of a resolution. Objects are still emerging from beneath Mexico's modern capital as part of the excavation. With luck, those objects will have their own stories to tell. If so, it may well be that the mysteries lying under centuries-old dust and rubble explaining the character of the great Aztec king, who had the privilege and misfortune to rule at the cusp of a new era, may yet be fully revealed.

The exhibition runs from 24 September to 24 January 2010. For tickets call 020 7323 8181 or book through the British Museum website