Adha

Adha is a character in Assassin's Creed: Altaïr's Chronicles. She is the Chalice itself, a sacred artifact like the Piece of Eden.

Assassin's Creed: Altaïr's Chronicles
In Altaïr's Chronicles, Adha is a woman revealed to be the Chalice, a holy artifact able to unite people. When Altair identifies her as the Chalice, he is absolutely surprised, as he apparently knew Adha before the events of the game. She, too, knew his name, so they probably at least heard about each other. Altair also expresses sympathy to her, suggesting him at least once encountering her in the past.

She is originally at Jerusalem, under Templars' watch, and she is rescued by Altaïr. He then learns that the Templars have paid off Harash, the second-in-command of the Assassins. Altair plans to attack Alep, the assassin fortress, kill Harash and run away with Adha, but after killing Harash, Adha is kidnapped by Basilisk and taken to their port in Jerusalem. Altair fights through the Templar knights and kills Basilisk. But Adha is on a different ship, which escapes before Altair can catch it. He swims to shore and, seeing the Templar ship far off in the distance, yells "I will find you, Adha!".

Assassin's Creed
Although she does not appear in Assassin's Creed, a mission giving Assassin at Damascus asks Altaïr if he heard from Adha. While Altaïr remain silent (as always with investigation mission giving Assassins), he says he is sure that one day she will come back to him.

Assassin's Creed II
Altaïr's codex reveals that he later caught up to Adha and had found her corpse. In the text, Altaïr states that he held her corpse and exacted vengeance on those that killed her. It is hinted that he loved her, and her death deeply hurt him. The circumstances of Adha's death are not made clear.

Her role as a Piece of Eden
According to Al Mualim in Altair's Chronicles, whoever has the Chalice can unite all the factions of whoever has it, giving them victory over all and allowing them to rule the land. If she is indeed a Piece of Eden, it may be possible she is one of Those Who Have Gone Before or may be 'created' by them.