Christoffa's atlas

Christoffa's atlas was a collection of maps in the possession of the Genoese explorer Christoffa Corombo, who used it in his voyages to navigate his way across the Atlantic Ocean. The atlas was unprecedented in its illustration of not only a safe, western route from Europe to the Orient, but also the continent of the Americas, then unknown to the Europeans. For this, it was regarded as priceless by not only Christoffa, but also the Assassins and the Templars.

It was compiled from the map scrolls of various cartographers, most notably Piri Reis, a Turkish contemporary of Christoffa who would later join the Assassin Brotherhood.

History
In 1491 or prior to that year, the atlas was acquired by Italian merchant and navigator Christoffa Corombo. From it, Christoffa drew much of his hopes for an expedition across the Atlantic Ocean, establishing a trade route from Europe to India and China that can circumvent the expensive middlemen along the Eurasian route. Much to his frustration, the monarchs throughout Europe did not share his confidence and repeatedly rebuffed his requests for funds.

By 1491, the Templars had managed to learn that the atlas revealed the existence of a giant landmass across the Atlantic. Aspiring for a head-start in its conquest, they sought to murder Christoffa, seize his atlas, and send their own expedition there before any other Old World power. Years of lobbying from 1485 came to naught, and the desperate Christoffa was lured to Venice in 1491 by Rodrigo Borgia, Grand Master of Templar Order, under a false proposal to sponsor his voyage.

Thanks to the foresight of his friend and primary benefactor Luis de Santángel, Christoffa was saved by the Assassin Ezio Auditore da Firenze. Nevertheless, he and Luis returned to their hostel in Venice to find that it had been raided by Borgia soldiers, as the Templars scoured its premises for the atlas. Once again, they deferred to Ezio, who sneaked into the occupied hostel and recovered the atlas from the bookshelf. After escaping the Borgia soldiers, Ezio returned the atlas to the arms of an excited Christoffa, who had been awaiting at the harbor with Luis in preparation for their departure.

Despite further Templar attempts to thwart Christoffa's expedition for the rest of the year—including a convoluted plot to prolong the Granada War and exhaust Castile's treasury from funding it, his voyage would proceed without issue in 1492. Though Christoffa's journey was ostensibly to chart a novel route to the Orient, in truth, the atlas he held had already had it charted for him. Furthermore, he never traveled the entire length of the way to Asia, landing in what would be dubbed the West Indies, and returning to Europe to propagate knowledge of the great landmass that would be called the Americas.

Trivia

 * The illustration of the Americas in Christoffa's atlas heavily suggests that, unlike in real-life history, Christoffa would have had foreknowledge of the continent before his first voyage. It also suggests that whereas the historical Christoffa had grossly miscalculated the circumference of the Earth, hence why he believed that such a voyage was feasible, the Christoffa of the Assassin's Creed series had a very accurate understanding of the Earth's geography. Nevertheless, it is never expressly stated in Assassin's Creed II: Discovery that Christoffa was aware of the unknown landmass, and he instead attributes its worth to the routes charted to Asia when describing it to Ezio.
 * In Assassin's Creed II: Discovery, map scrolls of the atlas are also presented as collectibles. The ten map scrolls found throughout the game each unlock challenge missions set in the Animus Virtual Training Program.

Reference

 * Assassin's Creed II: Discovery