From House of Hozz
In TwinWorld, rocks and metals are attacked by fungus just as readily as ripe fruit is in our world. These fungus fall under the general name of "stone rot", or just "rot".
Stone rot converts hard materials into softer materials. Hard stones are broken down, not mechanically, but magically, into softer stones, clays, soils, and eventually dust. In this way, stone rot is essential to produce most of the surface materials in TwinWorld. It also accounts for the lack of naturally occuring super ores on the surface. Stone rot attacks harder metals and stones more readily and more quickly than softer ones.
Stone rot proceeds through the bulk of a material. When it reaches the surface it concentrates the energy gained by breaking down the stone and generates spores. These spores are extremely hardy, and can lay dormant practically forever until they alight on an appropriate sample. At which point the rot begins to spread again. Many strains only consume a particular type of stone or metal, and stop once they have consumed the material in the immediate vicinity. By examining the residual stone-rot spores, one can somtimes learn what the original material was, and how many transformations it has undergone.
The fruiting bodies (spore generating structures like mushrooms or boils) of stone-rot are composed of the same material that the rot naturally produces. This leads to rot-growth, as the fruiting bodies add to the bulk of the rock they just broke down. The decomposition of the highest strength stone can result in double or triple the volume of fully weakened earth.
Stone rot uses a kind of natural transmutation magic, and the fungus is itself highly magical. Some strains are intentionally bred on blocks of stone and the fungal spores collected for use in enchantments. Because of the prevelance of stone-rot and the related spores in Zarthaanc there is an abundance of stone-rot spores continually pouring from Turmoil into the surface air. These are a common source of power and food for the Fay.
Stone rot can also be directed with wielding to preferentially weaken materials. Kept properly under control it can be used as a tool to carve stone and metal. Often the effort to direct the rot is less than that to shape the material directly. Rot-formed artifacts can be easily identified by their stippled surface. Since rot-forming is cheaper than direct wielding, many of the lower Zarth live in rot-formed dwellings. The disadvantage is that if the residual stone rot is not completely eliminated then the rot may re-emerge at a later date, damaging or destroying the artifact.
It has been employed as a weapon from time to time. Mostly of strategic value, as it is not usually fast-acting enough to be of value in a pitched battle. However, some strains can attack super-hard materials used in high performance weapons on the time scale of minutes. Zarth in combat must always be wary of the state of their gear, especially when rot bombs are going off.
Stone rot originated with Tanodia's second self-rune. A variety of strains are naturally present in the air that comes up from the source of all air. These strains change from time to time. Occasionally a very aggressive strain will wreak havoc in Zarthaanc as it converts the solid rock into brittle chalk which disintegrates in Turmoil and is carried away on the winds.
Over time, the Zarth developed and cultured several strains for forming and destroying materials. Any strain that is not captured and cultured is eventually carried by air and earth to the Horizon. Most recently it was a factor in destabilizing the Great Tree when a Zarth incursion into the surface was driven off by Fay sappers seeding a particularly virulent strain of stone rot.