Dungeons - and these things are also suitable for other locations such as castles, temples and manor houses - have walls, floors and ceilings. Such areas may be plain but, especially in important places or where the owner has a lot of money, they may also be decorated.
Such designs can come in different types and will usually be both large and epic in nature. They may even be valuable but, as they are built directly into the surface, players may find them difficult to remove, transport and sell. The different images described can be used to add dressings to a scene, make players think something is important or actually be used to hide clues and hidden things. The images range from the pleasant to the horrific to the peculiar.
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Here is a sample result:
An unholy figure in horned black armour is leading monstrous and deformed troops. The troops appear to be creatures from known sentient races, but all have been deformed in some way, with extra or missing limbs and heads, parts from other species, especially insect parts and tentacles and more sensory organs than they should have, including additional eyes, noses, ears and mouths in odd places. The mutations mean that they are wearing patchwork armour at best and many lack even that.
One page is the front cover, one the front matter and around two pages are blank and the introduction.