A Casket of Souls is not borne, but is summoned by an incantation. As its Keeper intones the proper chants, a fountain of skulls bursts from the ground below, revealing the Casket atop a dais of bone.
Once a Casket of Souls has been placed on the battlefield during deployment it cannot be moved by its crew during the Remaining Moves sub-phase.
Note that a Casket of Souls can still pivot freely at any time during its turn (the better to face the enemy) and may make a follow up move as normal.