Southern magic

Southern magic was a term used to describe unique spells written in a different magical language.

Origins
As the named implied, southern magic originated in south Faerûn, specifically Halruaa, Mulhorand, Unther, and parts of Chessenta. Wizards who were not native to those lands could not read spells or scrolls written in the southern magic language, even through the use of a read magic spell. The distinctive script was originally developed by priest-mages of Thoth to protect their secrets after the secession of the former Mulhorandi province of Thay. Therefore, it became also known as Thoth mage-script.

Rumors and Legends
In the mid-1360s DR, the brilliant wizard Random Riellor successfully created a spell that allowed non-Halruaans to decipher a spell written in southern magic. This spell was called read southern magic. Random only shared this spell with those he believed would help further his research.