Maledictus

A hereditary curse, affecting only human females.¹ This curse causes the victim (and her female decendants) to, over time shift from an animagus-like state of being able to shift to and from an animal form at will to being irrevocably transformed into that animal. As the curse progresses, the transformation becomes increasingly less and less voluntary, particularly when asleep. Despite having the form of an animal, the victim retains a human lifespan.

It is important to note that not all victims become snakes, nor do all victims become evil.²