Ahghairon's Dragonward

Waterdeep is blanketed by an undispellable magical effect called Ahghairon's Dragonward. The effect originates from somewhere under Ahghairon's Tower in the Castle Ward and is permanent. Dragons and all other creatures of the dragon type are physically unable to enter the city (or its sewers) as long as the dragonward persists. The effect doesn't extend to the harbor or into Undermountain.
A creature of the dragon type that is touched by the Dragonstaff of Ahghairon can ignore Ahghairon's Dragonward and move through the city freely. The effect lasts until the creature is touched again by the staff, or until the passage of a period of time specified by one who is wielding the staff.
A dragon that is within the dragonward while they lose the effect of the dragonstaff is magically compelled to leave Waterdeep by any means necessary until they are no longer inside the dragonward.