Each Slann Mage-Priest must join a unit of Temple Guard when both are present in the same army. Each unit of Temple-Guard may only be joined by a single Slann. The Slann cannot leave this unit - the Temple Guard are spawned to protect the Slann and their temples, and they will not leave their charge for anything, even if the Slann might wish them to! While there is a Slann Mage-Priest within the unit, the Temple Guard are Immune to Psychology and Stubborn, as described in the main rulebook.