Found on a corpse on the Road of Sacrifices. From the Crucifixion Woods bonfire, head into the nearby ruins that lead to the Crystal Sage. Inside the first room with several Sage's Prentice and a Sage's Devout the staff can be found inside an alcove on the first floor next to the stairs.
The Heretic's Staff is notable for being the most powerful staff for casters with 40 or less Intelligence, having the best Spell Buff rating for casting Sorceries for much of this range, short of a temporary dip in the 30 Intelligence range. Beyond 40 Intelligence, it begins to lose out to other staves such as the Preacher's Right Arm, Sorcerer's Staff and Court Sorcerer's Staff.
For this reason, it is considered the ideal staff for casters who are still in the process of creating their builds and wish to deal meaningful damage in the meantime, or for hybrid/utility casters who wish to stop at 40 Intelligence or lower.
Reinforced with regular Titanite.