The Hydra

Unlock Slug’s cage in Warrens, the ogre will reveal The Hydra’s hidden location to the party as thanks for freeing him, who's been forced to do labor works and provide "special services" to crusader women.

In-game description
"The three heads of this flail look like dragon heads, giving the weapon its fearsome name. As the wielder swings the weapon, its heads seem to blur and divide until a mass of hydra heads whips from the end of the chain. When the wielder ceases attacking, the weapon reverts to its original appearance."