Lycanthropy
One of the most ancient and feared of all curses, Lycanthropy can transform the most civilized humanoid into a ravening beast. In its natural humanoid form, a creature cursed by lycanthropy appears as its normal self. Over time, however, many lycanthropes acquire features suggestive of their animal form. In that animal form, a lycanthrope resembles a powerful version of a normal animal. On close inspection, its eyes show a faint spark of unnatural intelligence and might glow in the dark.
A humanoid creature can be afflicted with the curse of lycanthropy after being wounded by a lycanthrope, or if one or both of its parents are lycanthropes.
Lycanthropes will typically either resist their curse, or embrace it. Those that resist it try to live life as they always have, burying deep the bestial urges raging inside them. However, when the full moon rises, the curse becomes too strong to resist, transforming the individual into their beast form--or into a horrible hybrid form that combines animal and humanoid traits. When the moon wanes, the beast within can be controlled once again. The creature may not remember the events of its transformation, especially if it is unaware of its condition, though the memories often haunt them as bloody dreams.
Some individuals see little point in fighting the curse and accept what they are. With time and experience, they learn to master their shapechanging ability and can assume beast form or hybrid form at will. Most lycanthropes that embrace their bestial natures succumb to bloodlust.