The first mention of this figure comes from Eto Yoshimura, who describes a hybrid that rose to prominence and led an uprising against humanity. Their battle led to the Japanese government granting authority to the Washuu Clan, creating the Commission of Counter Ghoul to crush the revolution. 
Nishiki Nishio describes another story of a One-eyed ghoul leading an uprising while humanity was recovering from the 2nd World War. This figure was stated to have created a major conflict against a weakened humanity, resulting in the creation of V. According to the legends, this individual was eventually defeated and forced to flee underground to the 24th Ward. Stories claim they founded a great city underground, where they would wait until the time when ghouls could again rise up against humanity. 
An expedition into the deepest sections of the 24th Ward, led by Ayato Kirishima, discovers what appears to be a city.
It is unknown whether these stories refer to a single individual, or multiple figures existing in different eras.