The demonic presence dubbed 'King of the Cave' that dwells within The Counte of Monte Cristo, and the driving force behind his revenge plot throughout the series.

Spoiler

The demon's origins are tied to the Chateau d'If where Edmond Dantes was imprisoned falsely, as he was calling out for death but gained Gankutsuou's power and drive and helped him to escape. Gankutsuou does fully manifest near the end of the series as a triple set of dual colored eyes that run down the greater length of the Counte's face, and is suppressed by a special drug.