Altaïr Ibn-La'Ahad is an ancestor of Desmond Miles, a modern-day bartender who is kidnapped by pharmaceutical conglomerate Abstergo Industries and forced into entering a machine called the Animus to relive the memories of his ancestors.
Altaïr was born in 1165 to his father Umar Ibn-La'Ahad and mother Maud, who died after complications from childbirth. After his father is executed by the Saracens for taking part in an assassination plot against the leader of Saladin, he was taken under the guardianship of Al Mualim, the Mentor of the Brotherhood of the Assassins, who trains him into a highly-skilled assassin in the Masyaf Castle in Syria. He initially gains the respect of his fellow Assassins after rescuing Al Mualim from capture, though gets stripped of his rank for later failing to recover an artifact known as the Piece of Eden from Solomon's Temple and the forces of Robert de Sablé, and inadvertently allowing Masyaf to be ambushed by the Templars, the lifelong enemy of the Brotherhood. Al Mualim stabs Altaïr as punishment, though tells him that he can regain his reputation if he manages to kill nine specific men that roam about in the Holy Land during the Third Crusade in 1191.
As Altaïr successfully eliminates each target one by one, he begins to learn that the nine are all members of the Templar Order who were conspiring to locate the Apple of Eden, the artifact he initially failed to retrieve, which is said to posses god-like powers. Altaïr eventually tracks down his final target at the camp of King Richard I, which turns out to be de Sablé in the middle of negotiating an alliance between the Crusaders and the Saracens against the Assassins. The two then enter into a duel, in which de Sablé is mortally wounded, though he manages to tell Altaïr that he did not act alone and that Al Mualim has betrayed the Assassins as he too sought after the Apple of Eden. When Altaïr returns to Masyaf, he finds Al Mualim already in possession of the Apple and has used it to control the Assassins and the civilians. With the help of several outside Assassins who have arrived to provide backup, Altaïr storms the citadel and engages in a battle with his former mentor. After he kills Al Mualim with his hidden blade, Altaïr tries to destroy the Apple of Eden, but he instead unlocks a secret map that reveals the locations of other Pieces of Eden artifacts around the world. He then burns Al Mualim's body and tells the people of his treachery. Praised a hero, Altaïr assumes the role of the new Mentor to the Assassins.
Soon after, Altaïr begins to chronicle his research with the Apple as well as his life in a journal known as the Codex. During this time, he uses what he's learned from studying the Apple into creating new techniques and weapons for the Assassins to use, creating a prosperous era for the Brotherhood. In 1222, he travels to Western Xia, China on a mission to assassinate Genghis Khan; ten years later, Altaïr returns to find that his control of the Order had been usurped by his rival and former childhood friend Abbas. Their resulting confrontation unfortunately ends with Altaïr's wife Maria getting stabbed, and he later flees from Masyaf with his son and grieves for years. Eventually, he returns to stage a coup within the Order to reclaim it from Abbas with the help of several other Assassins.
After retaking the Brotherhood, Altaïr worked to rebuild it by splitting it up into smaller guilds and spreading them throughout the world with the help of Venetian explorers Niccolò and Maffeo Polo, believing that the Brotherhood should not be sequestered in Masyaf. He also begins to construct a library underneath the citadel containing all of his knowledge; some of the library's books (including the Codex) and the memory keys to the library would be given to the explorers to escape to Alexandria with after Mongols attack the Castle. Altaïr then seals himself in the library with the Apple of Eden, bids farewell to his son, and inside writes one final memory on a secret key he kept with him, and later peacefully dies at the age of 92. His remains and the Apple wouldn't be found until the 16th century by his descendant, Ezio Auditore da Firenze (the protagonist of Assassin's Creed II, Brotherhood, and Revelations), where Altaïr's teachings and methods have since become widespread and multiple Assassin guilds established.
The sword it uses is the Sword of Altaïr; a silver broadsword with an eagle-shaped hilt that Altaïr used for much of his time. The sword would later be used by Ezio Auditore da Firenze after he finds Altaïr's remains during the Renaissance. In future Assassin's Creed games, the sword (or replicas of it) becomes purchasable in shops as a bonus or as an Easter egg.
- Oddly, in the North American upload of the "Mii Fighter Costumes - Round 5" trailer featuring Altaïr, it lists his game of origin as his name instead of Assassin's Creed. He shares this strange distinction with Vault Boy and Travis Touchdown.
- Altaïr's segment of the trailer make several references to the Assassin's Creed series:
- The intro with the Mii Swordfighter landing on the Loftwing's nest in Skyloft references the Leap of Faith, a famous technique used by Assassins to safely land on soft material (commonly piles of hay) after jumping from a great height.
- The scene where the Altaïr costume hangs from the edge of Gerudo Valley and then getting back up to Back Throw Sheik off the stage is meant to mimic the technique where Assassins pull their targets down from a high place to their death with the Hidden Blade.
- The Altaïr costume battling Robin is meant to mimic Altaïr's final battle against Al Mualim, as Robin's attire and Levin Sword match Al Mualim's appearance.
- Strangely, the song used in the Altaïr costume's segment in the trailer is "Ezio's Family", a track from Assassin's Creed II and Assassin's Creed: Brotherhood, despite Altaïr being the protagonist of the first Assassin's Creed.
- On the American Nintendo eShop, Altaïr's name was briefly misspelled as "Altïar". This has since been fixed.