Your army must include at least one Wizard that uses the Lore of Necromancy.
Slain General
At the end of the phase in which the General is removed as a casualty, and at the start of every friendly turn thereafter, all friendly Undead units on the battlefield must take a LD test. If the test is failed, the unit immediately suffers a number of Wounds equal to the amount by which it failed the LD test, with no saves of any kind allowed. These Wounds are distributed as if from a shooting attack.
If, at the start of any of your turns following the death of the General, there is one or more friendly Wizard Characters on the table who know spells from the Lore of Necromancy, no unit in the army needs to take this LD test while at least one of them remain.