(Note: Unlike other types of beastmen, such as merfolk, horned men, and even most serpentfolk, the werebeasts of the Black Steel world are clearly and presently magical in nature. While we rely on inference with beastmen of other types, there is no question that the very existence of werebeasts is attributable to the actions of wizards or pantheonic forces, as their transformations result from the action of enchantments that are discernible to anyone sufficiently practiced in the use of sorcerous vision.)
A thorough study of the transformation of humans into animals and the other way around would be the work of several lifetimes, and there are few workable definitions of the term werebeast that would shorten the task considerably. By the simplest and broadest definition, a werebeast is any creature under a spell that, when triggered, will turn it into a creature of a different species. In many cases however, such "werebeasts" are best considered as enchanted beings of a particular species, regarding the spell as an external force acting upon them. The exceptions are few as cases, but much more commonly found, because each case includes large numbers of individual werebeasts: Creatures that can pass on their enchantment to others by straightforward and repeatable means.
In Theril's estimation, the most common form of werebeastial descent is through the process of natural childbirth. Some werebeasts are capable of breeding within their own racial group, passing on their enchantment from one generation to the next, and there is speculation, but little evidence, that some are even capable of breeding with members of at least one of the natural species into which their enchantments transform them, and bearing werebeast children. Legends persist of means by which a werebeast can transform an already-existant being into one of its kind -- typically by "infecting" a victim with a bite, though other means appear in more unusual stories -- but Black Steel personnel have to date found no definite evidence of such an occurrence outside of folklore, and Theril doubts the long-term viability of a bite-propogated werebeast community. Indeed, the preponderance of evidence she has collected on groups of werebeasts in and around Grat'ha is highly suggestive of a family organization, with all the telltale signs of hereditary similarities. She grants the possibility that in more civilized lands, where werebeast communities of any kind are unlikely to prove lasting, "unusual" enchantments might be the norm among those rare cases where werebeasts appear at all, but remains skeptical of the idea of bite-based spell transmission, primarily because working a spell capable of performing such a feat would be difficult in the extreme, and would seem to serve no purpose that could not be more accomplished by simpler means. When the possibility of pantheonic intervention is raised in defense of the legends, Theril's typical response is, "Show me one case." So far, no one has met her challenge.
Some werebeasts show clear signs of rational thought in both human and bestial forms; recent experience in Grat'ha suggests that some are even capable of wielding sorcery. Others, in at least one form, seem to exhibit behavior consistent with their current animal species, or of a particular species into which they can transform. (Most werebeasts shift between only two forms, one of them usually human, but rare occasions have been reported of beings with three, or even more.) Yet others consistently act with such ferocity, rage, and violence as few creatures of the world ever sustain for long naturally, driven presumably by their particular enchantments, further clouding the question of who or what a werebeast truly is. As a result, any reasonably concise description of werebeast thought, culture, or society would be hopelessly misleading or incomplete, even if Black Steel personnel had any significant experience of studying such a society.
Among the popular myths about werebeasts are stories of transformations that wait for the light of the full moon, and about tremendous resiliance or resistance to harm in battle. Some say, for instance, that only a specially-blessed or silver-coated weapon can harm werebeasts. In some of these tales, there appear to be kernels of truth: Werebeasts are often larger, more cunning even when not actually rational, more ferocious, or otherwise more dangerous than their natural counterparts, and the act of transformation sometimes allows them to survive or more speedily recover than would a normal being from such wounds as they sustain. The notion that silver might be necessary or useful to defeating such creatures is strictly a northern myth, most likely spread by peasants who saw the gleam of a trained warrior's properly-upkept sword and attributed his success to his "silver" blade instead of his skill. When asked about specially-blessed weapons, Theril once responded, "It's quite normal for frightened people to trust in their priests and their faith to protect them, regardless of whether they can or do or need to help anything." Nothing she's seen since then has given her reason to change her stance. And as for full-moon transformations, the experience of Black Steel personnel has been that it is purest nonsense; some werebeasts appear to change forms at nightfall and revert at daybreak, and others to switch back and forth seemingly at will, but the time of month appears irrelevant.
