Witchoil (also called gloom oil) has the appearance of pitch black oil, often with motes of bright white light suspended within it. Witchoil can be refined to a variety of different grades, but a common property of the substance is that it can capture the souls of those who die near a sample of the substance. A soul-empowered sample of witchoil can be used in a variety of spells and rituals, and is also highly flammable (explosively so, with some grades of sufficiently empowered oil).
According to Wolfgang von Recklinghausen, witchoil can be refined by alchemists from the energy of the Bleak Gate that sometimes bleeds into this world, though this practice only produces very small quantities of the substance.
Former mayor of the Nettles, Reed Macbannin had discovered some way to produce vast quantities of the substance, likely enabled by the fact that the Obscurati have some means of traveling with relative ease between the living world and the Bleak Gate. A mishap at the witchoil production facility beneath Macbannin’s residence resulted in witchoil flowing into a few areas of the Nettles.
Since the destruction of Macbannin’s facility, small stockpiles of witchoil have been discovered throughout the city in the possession of various underworld figures.