The Hebi Gang (蛇) is an anarchist group lead by Kôzuke and rose to power roughly six years ago. They seek to tear the Blood King off his throne and replace him with the "true king."
Hebi, (蛇) literally means "snake," or "serpent."
Under the leadership of Kôzuke, the Hebi gang grew to such a level of power that they were able to amass a well-manned army, starting a war three years before the start of the series.
However, they lost this war, though at quite the great cost of the Blood King's side.
Their army is currently rebuilt and at the same level of strength as it was before.