Perhaps it starts as a person who gets a wishing ring and who wants to be a, I dunno, vampire. The ring turns the host into one until the ring is removed.
You can't store the ring: it melts into mist if it isn't worn for a week and it re-forms near someone with a hunger for a different (clichéd) form.
Stopping it forever is left as an exercise for the players.
Think of it as an excuse to run something classic as an opponent: vampires, kaiju, creatures from the black lagoon, pirates, gangsters, dragons...