Crystal Skulls
The Crystal Skulls are Pieces of Eden, created by Those Who Came Before. They were scattered throughout central America, coming in the possession of Indian tribes. They are mentioned briefly in one of Warren Vidic's emails in Assassin's Creed. In 1520, Giovanni Borgia managed to obtain one from an Aztec priest while being undercover in Hernán Cortés' crew.[1] .[2]
Mitchell-Hedges-Skull
Several of the skulls did presumably come into possession of Abstergo, as Alan Rikkin notes in an email to Vidic that the Mitchell-Hedges Communicators work. Hence it is assumed that the power of the Crystall Skulls is some sort of communication with each other. Therefore, Abstergo plans to use them as a safe communication channel.
According to the email, Vidic possesses one of Crystal Skulls.