Threwd, also known as the Horrors, is an abstract concept, a sensation of awareness that lies in both the land and waters. It is closely connected with monsters, which are able to generate it.
One theory about threwd is that the land is strangely self-aware and intelligent, and threwd is the feelings given off by the land that can be felt by monsters and humans alike. The former however, are much more sensitive to it and can discern the nature and intent of threwd, such as whether it waxes or wanes, or is benevolent or hostile. It took a while for Rossamünd Bookchild to realise that his sense of threwd was much more acute that than of other people, as he is a rossamünderling.
It is well known that the most threwdish places are favourite haunts for monsters, and some theories even claim that there is some beneficial relationship between monsters and threwd. Some go as far as saying threwd creates monsters as a defese mechanism to protect the land from humans.
- Paltry threwd: The mildest kind, which can make a person feel uneasy, as if being watched, or feel like there is an unfriendly presence in an area.
- Pernicious threwd: The worst kind, which can drive a person completely mad with unfounded and sometimes founded fears and paranoias. Many expeditions sent to "tame" pernicious threwd and make land suitable for habitation have disappeared entirely.