Riou

Riou, also called Leo in the manga translations, and Banikis Gigo (his partner) are the main antagonists of the first half of the Faudo Arc. Riou utilizes Beast Magic, which is similar to some of the final spells of many mamodo that are in the forms of animals.

Riou also has the ability to curse humans, causing them to become weak and eventually die. Riou's will is the only thing that can break the curse. The curse has been passed down through his family for ages, for reasons unknown. Although, using this spell also puts a strain on his own body, especially when used on many people. He uses this ability to receive help on breaking Faudo's seal by making a few Mamodo bookkeepers help him to free Faudo, or die if they didn't. The humans that were affected by this curse were Nicole, Ellie, Alishie, and Li-en.

(Source: Zatch Bell! Wiki)