Every sorcerer or wizard of Faerûn who desires to do so can adopt personal rune or mage sign that marks that mage's work. These are sometimes simply marks of ownership or territory, but it is best to assume that every last rune graven on a doorway or boundary stone holds at least one spell waiting to be triggered - usually by handling the item or casting magic upon it. Triggers may be specific to race, gender, time, and location of the rune-marked itens, presence of other substances or magic, and the like.
Spells so hung are usually of a guardian nature, but may do great harm. Explosive runes, sepia snake sigil, fire trap, and symbol of death are the most common, and rumors abound of stranger and more deadly spells that have been devised to guard a mage's rune.
No two mage runes are identical. A design chosen for a rune simply won't function as such if it accidentally resembles one used by another mage (either active or in the past). Most who cast spells in Faerûn choose one rune for their entire lives, never changing.
By the will of divine Mystra herself, the High One visits a heavy curse upon those who falsely use the rune of another creature. As the old apprentice rhyme reminds us:
Whenever magic one doth weave
'Tis never, ever, wise, to deceive.
- Elminster of Shadowdale
All arcane spellcasters create a personal sigil or rune. This rune is used to identify belongings, as a warning and as a signature. As the goddess of magic, Mystra takes it upon herself to protect powerful spellcasters (above 10th level) maligned by another using their rune with deceitful intent. If anyone (spellcaster or not) willfully copies a sigil with the intent to trick another or usurp the owner's identity, that person must make three DC 15 Will saves to avoid the trifold Curse of Mystra.
Magic in the Realms