The Pyramid was a device created by the Isu capable of computing possible futures. The Pyramid was made up of many triangular segments and also possessed the power to observe a person's memories when they touched it if they had Isu heritage.
By the 5th century BCE, the artifact was owned  by Pythagoras' followers who had formed the Cult of Hermes and worshipped the Greek god Hermes Trismegistus after their teacher met him during his travels. When a number of them grew dissatisfied with the group's ways, they formed a splinter group and called themselves the Cult of Kosmos, choosing to worship an undescribed entity that they saw as the personification of chaos.
The Cult then stole  the Pyramid and moved it to the Sanctuary of Kosmos, located beneath the Temple of Apollo in the Sanctuary of Delphi From their new base, the Cult intended to manipulate reality and control the 5th century Greek world, using the artifact's calculations to make prophecies for generations of Pythias to profess that were most beneficial to their purposes. This continued until 480 BC, when the the Cult kidnapped and raised the Spartan infant Alexios to be the Cult's enforcer "Deimos", since his direct descent from an Isu bloodline through his grandfather Leonidas I allowed him to access the artifact's powers more easily.
Decades later in 431 BC, however, Deimos destroyed the Pyramid in a fit of rage during the Cult's first meeting in many years. Upon learning that the Kirrhan Cultist Elpenor was murdered, Diemos angrily stormed back to the Sanctuary and began forcibly interrogating the masked group by using the artifact to see their memories and root out the traitor. Unbeknownst to him, his sister Kassandra had infiltrated the meeting in Elpenor's robes, and when Deimos called on her, the artifact showed them both that their long-lost sibling had survived their childhood fall off Mount Taygetos. Shocked at the revelation, Deimos bade his sister to leave and called on the Cultist Epiktetos, only to inexplicably grab the back of the man's head and slam his face into the Pyramid's peak, instantly killing him and shattering the device into small fragments.
After Kassandra dismantled the Cult, she and the former Cultist Aspasia received different messages from the artifact and each travelled to the Sanctuary of Kosmos. Once Kassandra saw the message from her father and a vision of future Assassins, such as the Egyptian Hidden One Bayek and the Italian Assassin Ezio Auditore, she stabbed the Spear of Leonidas into the center of the empty pedestal that projected a light construct of the pyramid, destroying its ability to power the artifact shards.
Behind the scenes
The prophetic vision received by Kassandra is in fact a combination of cinematic trailer footage from several previous games in the series.
- Assassin's Creed: Odyssey (first appearance)
- The Art of Assassin's Creed: Odyssey
- Assassin's Creed: Odyssey – The Serpent's Lair
- Assassin's Creed: The Essential Guide
- Assassin's Creed: Odyssey – A Fresh Start
- Assassin's Creed: Project Legacy – Divine Science: Chapter 2 - Kyros of Zarax
- Assassin's Creed: Odyssey – Cult of Kosmos clues: Zoisme's Journal
- Assassin's Creed: Odyssey – The Big Break
- Assassin's Creed: Odyssey – Consulting a Ghost
- Assassin's Creed: Odyssey – The Truth Will Out
- Assassin's Creed: Odyssey – Sins of the Past
- Assassin's Creed: Odyssey – Bully the Bullies
- Assassin's Creed: Odyssey – The Priests of Asklepios
- Assassin's Creed: Odyssey – Memories Awoken
- Assassin's Creed: Brotherhood – Modern day