Sacred Duty
(Rule)Description: If both a Slann Mage Priest and a Temple Guard are in the army, then the Slann must be placed in the unit.
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.
Whilst there is a Slann Mage Priest within the unit, the Temple Guard are stubborn (page 85 of the Warhammer rulebook).