Adha

Adha was a young woman also known to be "the Chalice," a sacred artifact similar to the Pieces of Eden.

The Chalice
The Chalice was known to either be a holy artifact, or the descendant and heir of Jesus Christ. According to Al Mualim, whoever had the Chalice could unite all factions under them, granting them victory over all, and allowing them to rule the land. Seeing the danger of such an artifact, the Mentor sent one of his Assassins, Altaïr Ibn-La'Ahad on a mission to retrieve it.

Though his search led him to the Temple of the Sand, there, Altaïr only discovered an empty golden chest. This prompted him to assume that it had already fallen into Templar hands, thus he traveled to Jerusalem.

Rescue and Capture
Discovering that the Chalice was being kept under Templar watch in the Holy City, Altaïr infiltrated their fortress and soon found Adha. Though he was absolutely surprised upon identifying her as the Chalice, he rescued her nonetheless.

Shortly afterwards, Adha informed him that the Templars had bribed Harash, the second-in-command of the Assassins, to betray their Order; thus Altaïr decided to seek him out at the Assassin fortress of Alep. Though he planned to kill the traitor and run away with Adha, upon completing his task, Adha was kidnapped by Basilisk, Grand Master of the Templar Order.

Pursuing them to a port in Tyre, Altaïr fought his way onto Basilisk's ship at the head of the fleet, killing several Templar knights and Basilisk himself. However, he discovered that Adha was being held on a different ship, which escaped before Altaïr could catch up to it. He returned to shore and, seeing the Templar ship far off in the distance, called out to Adha, promising that he would find her.

Aftermath
"I had thought Adha would be the one to lead me to rest, that I might lay down my blade and live as a normal man. But now I know such dreams are best left to sleep..."

- Altaïr, in his Codex.

In 1191, during Altaïr's preparations to assassinate Abu'l Nuqoud in Damascus, an Assassin informant asked him "Any news of Adha since she left?" When Altaïr remained silent, as always with informants, he continued with "No? How sad. I'm sure you'll find her someday."

Though the circumstances of Adha's death are unclear, Altaïr wrote in his Codex that he later caught up to Adha, but only found her corpse. He went on to state that he had hunted down and exacted vengeance on those that had killed her. It is hinted that he loved her, and that her death deeply hurt him.

Trivia

 * Though not explicitly mentioned, the fact that Altaïr and Adha know each other by name upon first meeting in Assassin's Creed: Altaïr's Chronicles suggests that they have met at least once before.
 * An e-mail in the computer at the Abstergo Industries conference room mentions the Chalice, and questions whether or not it is a real Piece of Eden. It also notes that the search for it would be called off until more evidence of its existence could be found.