The Wards of Silverymoon
The city of Silverymoon is protected by a powerful magical effect called a mythal. This potent form of elven high magic forbids the casting of several types of spells, causer other spells to, be permanently in effect, and allows the denizens of Silverymoon to use still other spells with a word. The mythal's effects cover the city itself plus an area 1,000 yards beyond its limits in all directions (even up into the sky and down into the earth).
Spells with the death, evil, or teleportation descriptor, conjuration (summoning) spells, and evocation [fire] spells are all forbidden by the mythal of Silverymoon. Any attempt to cast such a spell fails automatically, though the spell slot is used up normally.
The following spells are continually in effect throughout the area of the mythal, much like a spell fixed to a hallow or unhallow effect: antipathy (to all evil-aligned demons, devils, dragons, drow, duergar, giants, goblinoids, mind flayers, orcs, and trolls), detect scrying (all creatures within the mythal gain the benefits of this effect), invisibility purge, negative energy protection, and protection from evil.
Finally, characters attuned to the mythal may activate any of the following spells by command word, exactly as if it were an ability of a magic item in the character's possession: air walk, bless weapon, bull's strength, cat's grace, control water, discern lies, dispel chaos, dispel evil, feather fall, lesser ironguard, quench, remove curse, remove paralysis, shield, shocking grasp, silence, and tongues.
Becoming attuned to Silverymoon's mythal requires a special ceremony that lasts a full day, plus the expenditure of 2,500 XP. Attunement also requires the aid and approval of Azuth, Mystra, or one of the leaders of the city.
More information on Silverymoon's wards can be found on page 57 of The Silver Marches.