The Assassin's Creed franchise has story-rich lore, which is also easy to remember because of the key historical moments embedded within the narrative of each game. Arguably, Assassin's Creed Odyssey and Origins play the most important parts as they explain where it all began.
Assassin's Creed Odyssey
The game was set in Ancient Greece between 431 BC to 422 BC, which means that it was 1,600 years before the first game in the franchise. The player can choose between two characters to play, which are Alexios and Kassandra.
In the story, you are the grandchild of the most well-known Spartan, Leonidas I who is partly Isu (an important detail for later) and will fight various battles in the Peloponnesian War since you will choose sides between Athens and Sparta.
The base game in itself already explains how a group of powerful individuals could influence the way of the world. Odyssey introduces the Cult of Kosmos and their goal to control Greece for their own interests. In a way, they are guided by The Pyramid a device created by the Isu.
For context, the Isu are powerful and intelligent beings, regarded as The First Civilization or "those who came before." These are the gods known in Greek mythology such as Zeus, Poseidon, Apollo, Artemis, and Hermes, among others.
The Pyramid is able to determine potential outcomes, which means that the Cult of Kosmos is able to see what could be if they choose to go down a certain path. Serving as a weapon to the Cult is Alexios or Kassandra (depending on who you choose).
Leonidas' bloodline becomes important since it can be used to interact with Isu devices, which is one of the reasons why they're keeping the main character's sibling at arm's length. In one of the storylines, Kassandra/Alexios finds Pythagoras, who is their biological father.
This is where the first Isu artifact which is the Staff of Hermes Trismegistus, is revealed, also referred to as the "Apple of Eden" in previous games. However, you will need to play the DLC to understand the main connection between the stories.
The Legacy of the First Blade DLC explains how such a national superpower like the Cult of Kosmos is linked to the Templars. Having the same goal, the Cult aligns with another organization called the Order of the Ancients (which eventually becomes the Templar Order).
Other than the DLC that introduced the Hidden Blade, it also tells the story of how Kassandra eventually had a child named Elpidios. Since he would also be hunted because he is a "Tainted One," Kassandra decided that Darius (grandfather of Elpidios) should take him away.
Assassin's Creed Origins
In an effort to bring Kassandra's son to a safe place, Darius eventually settled in Egypt. Elpidios grew up under his grandfather's care, eventually having children of his own. His descendant, Aya, becomes the partner of the Assassin's Creed's main character, Bayek.
Their mission begins when Aya and Bayek's son, Khemu, was killed. On the path to avenging their son's death, they come across the same organization that aligned with the Cult of Kosmos, the Order of the Ancient. Like Kassandra/Alexios, they hunt the members one by one.
It's unclear whether Aya gave birth to another child, but if not, then the Hybrid line (Isu and Human) ends with her. It's an entirely different topic to discuss, but long story short, hybrids are the descendants of Adam and Eve, the first hybrids that cannot be controlled by Isu artifacts.
Bayek and Aya realize at there will always be powerful and corrupt figures, so they formed the Hidden One to protect the innocent. The organization eventually evolved into what is more known as the Assassin's Brotherhood or Assassin's Order.
