Codex: Witch Hunters is an expansion codex book for Warhammer 40,000 3rd Eition. It is one of three books either published or proposed about the Ordo Hereticus (Codex: Daemonhunters, Codex: Witch Hunters, Codex: Alienhunters).
From the back cover:
The Witch Hunters of the Ordo Hereticus are perhaps the most sinister of all Inquisitors as the threats they must face lurk not in the depths of space or the Warp, but amongst the everyday citizenry of the Imperium; the witch, the mutant, the heretic and the fallen cleric. Such abominations are an affront to the Emperor and together with their Chamber Militant, the Sisters of Battle, they purge evil from the worlds of Man with the cleansing fire of purity.