Tombstones were enchanted items that kept the undead buried below in a state of hibernation. The elder was an extremely powerful necromancer, possibly the best necromancer in mankind, but his power was still limited and controlling an undead army plus a few dozen powerful undead was too much for him.
That's what tombstones were for. Her enchantments kept the undead hibernating and lessened the burden they placed on her mind. Keeping the undead active even if they weren't doing anything put a burden on the necromancer after all.
To control an undead, a necromancer needs to subjugate its mind after creating it, but that doesn't mean the undead would become a vegetable after that.
Unlike the currents of Athos that altered the minds of the undead to make them loyal, normal necromancy erased all memories and personality from when they were alive, their minds regressing to the same level as newborns.
