[lch]

No, slaves can act on their own. The only thing is that if any one communion (sabbath etc.) master casts a spell in a combat turn, then they have to perform the "cast spell" action aswell, lending their support to the communion master, and can't act themself. But if all communion masters are somehow occupied, engaging in melee combat or already left the battlefield for example, then the slaves act as normal mages in a battle.

Take care that, as has been noted before, the actual fatigue that a unit gets is determined by the magic paths and levels he has as if he'd cast the spell himself (and then divide by number of slaves+master), while the communion master takes considerably less fatigue because he might be magically more powerful than the slaves. This is extra bad if the slaves don't have the actual levels required to cast the spell.