How do you find open slots? : rnationalguard

army slotI can't summon it using undead army? as in the 4th slot to add it is locked out. I can't find anything on the that points me in the rht direction.I can't summon it using undead army? as in the 4th slot to add it is locked out. I can't find anything on the that points me in the rht direction.