As should be expected of any big, sprawling franchise that's been around since 2007, Assassin's Creed has some pretty complex lore. While each game in the series focuses on its own story, set against its own unique historical setting and revolving around its own set of distinct characters, the majority of Assassin's Creed titles do share some similar plot threads. From the Assassin-Templar war, to Abstergo Industries, to the First Civilization, there's a lot of in-depth lore across the franchise, and it's easy to get lost in mysteries like The Grey.
One of the most complex and confusing plot points in the Assassin's Creed franchise revolves around the First Civilization. Also known as the Isu, these ancient god-like humanoids were the first sentient species on Earth, and were the creators of the Pieces of Eden. The Isu kept human slaves and eventually a war broke out between the two species, resulting in the Isu's extinction. But despite losing their physical bodies, some Isu have managed to live on in what is known as "The Grey."
Juno and The Grey
Though they were mentioned briefly in the very first Assassin's Creed game, the Isu were first seen in Assassin's Creed 2, when a projection of Minerva speaks directly to Ezio in the Vatican's hidden underground vault. While the Isu played a significant background role in the present-day sequences in the next few games, the first mention of The Grey wouldn't occur until Assassin's Creed 4: Black Flag.
Back during the human-Isu war, three important Isu were tasked with saving the Earth from the conflict. On this council were Minerva, Jupiter, and Juno, the latter of which had her own plan to conquer the world. Finding out about this plan, Minerva imprisoned Juno and created The Eye, a device that could alter reality and time, in order to save Earth. However, Minerva discovered that Juno had sabotaged The Eye, and activating it would release her from her prison. Refusing to release her, Minerva destroyed The Eye.
Thousands of years later, Desmond Miles locates a hidden Isu temple, and visions of Juno help guide him to open the gates. Inside, Desmond found The Eye, which Juno had fixed during her imprisonment. Juno asks Desmond to activate The Eye, telling him that it's the only way to save the Earth from the impending solar flare. Before he can do so, Minerva projects a vision to him, asking him to let the Earth fall, and keep Juno imprisoned. Deciding to save the Earth, Desmond activates The Eye, and releases Juno.
When Abstergo comes to inspect the scene and recover Desmond's body, Juno transfers her consciousness into the equipment they brought with them, and when they've returned to Abstergo headquarters, she uploads herself into the Abstergo mainframe. Now part Isu, part AI, Juno is able to gather information from across the world instantly, and launch a series of digital attacks. Juno calls this state of being "The Grey." Initially seeing her digital state of being as a disadvantage, Juno makes plans to transfer her consciousness into a human body, though her plan is soon sabotaged by Assassins.
After the failed plan, Juno changes her outlook, and starts to believe that The Grey offers a higher level of consciousness than the mortal plane does. Juno then hatches a plan to reward the best of humanity by transferring their consciousness to The Grey, though her plan is foiled once again. While Juno hasn't accomplished everything she intended to, The Grey could still come to be an interesting way to view other Isu characters in future Assassin's Creed titles.