Full Possession Mode is a state unique to the Masoukishin mecha of La Gias. Using the Spirit Dependancy Link, the Herald (pilot) can call upon the Masouki's respective Elemental spirit for increased power, abilities, and upgraded weapons.
The machines weren't built to withstand this mode, so the spirits even bestow a special armor to compensate. This lasts for a brief moment, as the mode not only drain's the Herald's prana, but can potentially kill them. The Cybuster is unique in this case, since its Laplace's Demon computer can interface with the Spirit Dependency Link, allowing Cyfis to grant her herald ability to access to the Akashic Records and gain control over the Principle of Cause and Effect, on top of the powerup that Full Possession usually grants.
.png "Cybuster (Possessed).png (399 KB)")
The Granzon has something similar to the Full Possession Mode. Using its Kabbalah System, Shu can tap into the powers of Volkruss. However, the Neo Granzon seems to be a more permanent transformation, unlike Full Possession, which lasts for a few minutes.
