At the Liche Priest's command, the sands begin to shift and churn, revealing their hidden treasures: the dry bones of long dead warriors and the glistening wing cases of biting insects.
Source: Warhammer: The Old World Online Rules Index
From Beneath the SandsURL Copied!
During the Command sub-phase of their turn, if they are not engaged in combat, this character may choose a single friendly unit that has both the Nehekharan Undead and the Ambushers special rules, and that is currently held in reserve, and attempt to summon it by making a Leadership test (using their own Leadership):
If this test is passed, the chosen unit is successfully summoned and can be placed on the battlefield anywhere completely within 12" of this model, but not within 6" of any enemy models. The unit cannot charge during this turn and counts as having moved for the purposes of shooting, but can otherwise act as normal.
If this test is failed, the Ambushers pay no heed to this model's summons and are delayed until their controlling player's next turn at least.
Note that Ambushers arrive automatically at the start of round five as usual.