When he was a child Daniel was kidnapped by Abstergo Industries. They dubbed him Subject Four and forcibly explored his genetic memories hidden within his DNA. Unbeknownst to him they secretly planted an impulse in Daniel, an impulse to kill the Grand Master of The Assassin Order if he ever got the chance. Abstergo later threw him out presumably because they were unsuccessful in finding a Piece of Eden. In 1998 he was found by The Assassins and inducted into their ranks.
Daniel Cross is the modern day descendent of the Russian assassin Nikolai Orelov. Cross is a recovering criminal drug addict who sees "visions" of his ancestor, which leads him to be sent to a court-ordered psychiatrist. After visiting a local bar he began to suffer from the hallucinations, stepping outside he bumped into a man an started accusing him of being Templar. Cross attempted to stab the man but was stopped by Hannah Mueller. Cross is later found by the modern day Assassins and is inducted into their ranks so they can use his genetic memories to learn more about Orelov's past, preparing them for the war against the modern day front for the Knights Templar, Abstergo Industries. Unlike the others before him Daniel does not have to use the Animus to pick up the skills of Nikolai, he is able to via a natural bleeding effect.
After being brought to the Assassin's hideout by Hannah Mueller, Cross experienced another vision and began to speak out the words of his ancestor, Nikolai Orelov. He began to speak of the Tunguska explosion, as he was peaking about he was questioned by Paul Bellamy, the head of the camp where they are staying.The visions ended and he got up to leave but was stopped by two other Assassins, Daniel was quickly able to take them down and as a result Bellamy had to use his hidden blade on Daniel to stop him from killing anyone.
That night Daniel seemed to be in rage, overturning his furniture in the process, catching the attention of Hannah. He was able to convince her that he left his medication at his flat. The two arrived at his apartment only to realize that he had thrown all of it out. She asked to let out whatever was bothering him to get it off his mind.
After briefly discussing a few things Hannah asked him about the hallucinations and what he sees during them. Daniel got up and walked over to the window, only to draw a picture of The Staff. A moment after Bellamy burst into the room, telling the two that in 1908 a team was sent to Tunguska to investigate and retrieve The Staff. Only had survived, Daniel's ancestor Nikolai Orelov. upon hearing the name Daniel had a hallucination taking him back to Nikolai's mission to Tunguska to retrieve the Staff. Bellamy urged Cross to tell him what he saw but all he got from him was punch to the chin. Daniel fled the apartment, made his way up to the roof, all during which he was suffering from the hallucination of his ancestor. Bellamy and Hannah pursued but by the time they caught up to him Daniel was collapsed on the roof. While he was running he saw what he thought was the climax of the vision, he saw The Apple of Eden, Nikola Tesla, Eden, and even his mom giving birth to him. His collapse was due to the images being too much for him to handle at once, the only thing he knew was that he had to find The Mentor, the Grand Master of The Assassin Order.
He would relentlessly search for The Mentor for the next two years. On November 5th his hard work paid off, he finally found The Mentor just not he expected to. While in apartment he attacked by two masked individuals. They overpowered him and then injected him with a sedative, and took him Dubai to the home of The Mentor. Waking up unsure of where his or who was speaking to him Daniel quickly deduced that he was in fact in the presence of The Mentor. He realized they had been following every step of his for years, watching him carefully. The Mentor explained to him what the title of The Mentor means, it was the title of all his predecessors and he intended Cross to be his successor.
He was given the gift of the Hidden Blade, the weapon of The Assassins. It was rarely used by the Assassins anymore but was still in working condition. He placed on his arm and felt as if he born to wear it. All of a sudden a strange impulse came over him, an impulse that was implanted in his subconscious many years ago by Warren Vidic, the head of the Animus project. Acting on his impulse he murdered the Mentor, regretting it but he knew that he couldn't stop himself from doing it. Hearing voices outside the room Cross had no choice but to leave and the was only one other way out the room, he leapt out the window to the water below. It was called a leap of Faith, something The Assassins perfected years ago.
In late November Daniel infiltrated an Abstergo Industries facility in Philadelphia, he was unsuccessful in finding anything as he was caught. Upon his capture Daniel demanded to go into the Animus, Vidic agreed and allowed him to "freely" roam through his ancestor's memories. For some unexplainable reason the Animus had a rather unique effect on him, it calmed him down unlike what it would do for Subject 16 in years to come.