The Ratwood Tree is a small and unassuming tree that seldom grows more than 30 feet tall. The Ratwood has a very dark hued but very pulpy wood. This wood isn't useful for making anything other than paper as it is soft, splintery, and given its natural oily resin foul smelling and smokey when burned. The leaves are large and dark colored, and the tree creates a large amount of solid shadow around its base.
The tree gains it's name from it's rather peculiar and actively hated fruit. The fruit of the tree resembles a rat dangling from a branch by it's tail. The fruit is able to move like a rat once it matures and falls off of the tree. These rat like seed pods are capable of running around, scurrying, foraging, and otherwise engaging in ratlike behaviour before falling dormant to begin the process of germination.
These rat pods seek out warm, moist, dark places and will create a nest and burrow down into it. This nest will serve as an initial cocoon for the seed which in short order germinates and begins the process of growing into a new ratwood tree.
The mobility of the ratwood's seeds means that the tree is able to scatter its seeds without relying on wind, luck, or herbivores. This ensures that a proportionately large number of ratwood seeds survive to germinate compared to other trees. A second survival technique is that the rat pods have a limited ability to 'eat' and extend their active phase. Their main source of fuel during this burst of activity is most commonly other seeds. This thins out competitors from other trees. It is common for established ratwood groves to contain exclusively old growth trees predating the appearance of the ratwoods, and ratwoods and nothing in between.
Ratwood seeds are drawn to human habitations where they will find their way to the same places that normal rats seek out. This is only a marginal problem as the seeds lack parasites and disease, but can be a real problem when a house becomes unstable because a large number of trees have rooted and are growing out through weak points in the foundations and the walls. In smaller and regularly inspected locations this isn't really much of a problem as the fast growing tree can usually be found before it does too much damage. In more remote, and less frequently inspected locations, such as ruins, wizard's towers, decrepit fortresses and abandoned towns, the trees can do 20 years of plant damage in 3-5 years.
Ratwoods are normally minor pests, causing structural damage, and some grain loss. Eventually, there can grow a situation where there are more trees, and consequently many more seed pods than can be sustained in the local area. The end effect is that the seeds will begin swarming, and will form raids away from the copse/forest that dropped them. These rivers of woody rat seed things will move in random directions, often tumbling into water where they are washed downstream. These swarms can be devastating to the surrounding terrain as the pods are voracious eaters, and will mow down fields, and lacking seeds will tear apart fleshy plants, tubers, and other non-flesh organic material.
Swarm! - the PCs are caught in a ratseed swarm, and are hired by the locals to seek out where the seeds are coming from and cut down the Ratwoods and burn them. Problem is that the local druid is both a fan of the trees and likes both his woody rats and real rats.
Spell Component - given the animal like nature of the Ratwood seeds, they are sought after by wizards for either use in making plant animation spells, and scrolls, or for research into their origins.