Ruathlek

Ruathlek is the "secret language" or magical script of illusionists. Waterdeep is known to hold at least one library of books in this dweomer-guarded tongue.

History
Ruathlek is descended from the Netherese language. It's said that Leira, the goddess of illusion and deception, developed Ruathlek with some of the descendants of Netheril, the wizards of Nimbral. Unlike most illusionists, the people of Nimbral use Ruathlik as both a magical script and their mundane, everyday language.