The Warp

The mind is a delicate thing, and whether from spiritual healers in rural parts to specialized sages in Republic cities, humans and dwarves alike have long sought remedies for unseen ailments. The Warp offers a unique challenge, in that the cause is mysterious even after millennia of study and there seems to be no cure. Dwarves believe that the Warp is what condemned an entire House to madness and depravity, and so far only those who take the oath to join the Blackheart legion seem immune. The Warp can manifest anywhere, afflicting a dwarven fisherman who has never spent a day underground, despite the malady driving those who contract it into the lightless depths below the world.
The Warp doesn't appear to be contagious, and proximity to certain features does correlate to higher incident rates. Though more advanced stages have no cure, the initial signs can clue in observant healers and with the right intervention early on, the Warp can be avoided. Dwarves typically become insomniacs and photophobes, and manifest tics or odd speech habits that weren't there before. These persist for a couple of weeks before the Warp advances, at which point the afflicted dwarf will seek to get as deep underground as possible. Intervention includes meditation in sunlight and specially prepared incenses, but because the Warp often strikes again if it doesn't succeed the first time, even those cured of it will be watched more closely. Those lost to the Warp disappear in the dark, possibly uniting with House Hugjarr or possibly starving to death in the deeps, or some other, more grisly fate.
Though humans can manifest signs of the Warp, none have ever been fully lost to it. Only the initial signs afflict them, and the intervention treatments mentioned above can cure them easily enough.