If your army contains any Undead units, you must include at least one character that is a 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 Leadership test. If the test is failed, the unit immediately suffers a number of Wounds equal to the amount by which it failed the Leadership 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 Leadership test while at least one of them remain.
Battle Standards
In addition to the normal rules for the army battle standard, units of Undead within 12" of their battle standard suffer D3 Wounds less than they normally would due to the Unstable special rule, or following the death of the army's General.
Characters and Units
Characters who are not Undead may join Undead units despite not being Unstable themselves. Mounted Characters ignore the Undead special rule of the mount. If an Undead unit would get destroyed as a result of Unstable, any remaining character(s) will need to take a Break test as normal the same phase, using the Leadership modifier that they lost the combat by.