Creation of the Universe
Unicron & Primus were twins created by an all-powerful being at the dawn of time itself. Unicron notably was created to destroy that which is brother Primus created when it was those creations times to end. All this to entertain their creator. However, Unicron grew jealous and slighted that his powers were seen as weaker and began to make is way devouring any world in his path as he searched for his brother.
The main part of the myth is this all-powerful creator referred to as the One. He creates Unicron to destroy all things created by Primus when it is their time. However, he gets jealous of his twin thinking himself the lesser of the two brothers and begins to destroy everything created between him and his twin. Hoping to end his brother and force their creator to acknowledge him.
Primus and Unicron do seem to exist. At the edges of reality is a large planetoid that consumes worlds as they draw near but something keeps it from moving past the very edge of existence. Quintesson's state that their planet was destroyed and some mention it wasn't destroyed buy devoured, likely by Unicron. A sect of the Quintesson's led by Kledji was spared the fate of their world and came to Cybertron based on information Kledji declared he recieved in a vision.
Unknown at the start of the infant rebellion the glyphs on the Well of Sparks is the story of the twins and the warning that Unicron is still active while Primus slumbers still.