The Codex of the Damned is, essentially, one vampire's journal. The Wanderer, Mictlantecuhtli, recorded details of his Thaumaturgical research. He kept it with him at all times and Tzental Falls priests, in an attempt to make themselves immortal like their god, coveted the secrets that were contained within.
Every day when the Wanderer would lay, sheltered from the burning rays of the sun, one of his aides corrupted by the priest would bring the Codex to them. They were unable to decipher the mysteries directly and instead copied it symbol for symbol until they had a copy of the Codex themselves. Mictlantecuhtli never learned that the priests had made a copy of his codex.
When Mictlantecuhtli descended into the Ziggurat to slip into Torpor, the priest were entombed with him. With the priests dead, the copy was passed from owner to owner until it eventual appeared in Zurich during the Renaissance. Other copies had been made by then but with sloppy copying which changed many of the secrets held within.
No one really knows who first cracked the Codex. Some say the Tremere Inner Council of Seven managed the feat. There are thought to be two true translated copies of the codex still in existence. One in Latin, thought to rest in the protective hands of Clan Tremere in Vienna, the other in English which has vanished from Kindred eyes.