No Magical Lighting
Throughout the adventure, no source of magical light the characters brought with them will function: sunrods are inert, everburning torches have “gone out”, light spells simply fizzle, etc. Magical lighting found within the module (e.g. the braziers in the Abattoir work just fine, and may be taken by the group to other areas.
Most rooms have their own lighting in one form or another, but are typically no better than dimly lit. The characters can use any non-magical lighting they have, of course.
This typically creates a little trouble for the group, as it means taking up hands with torches, lanterns, etc. Feel free to play this up (at the most inopportune times…).