Mortallus

Through research in a library in Hygard, Tektite finds clues of "Mortallus" being some kind of Kuo-Toa Swamp God.

It is revealed that Tektite hid the true line he read in the library, which was, “Mortallus is clearly a legendary figure prominent throughout the western islands …” instead spouting that it was a Kuo-Toa Swamp God and proceeding to destroy the book. (S3 E7)

A figure with slit-eyes said "ahh, Mortallus" upon seeing Tektite being led out of a carriage in Frandelle. (S3 E11)

In the Frandelle History Museum, in Le Grande Drip, it mentions that sailors continue to spread the myth of Mortallus, the general attitude towards this is that, "it is silly". (S3 E12)