Overgrown Secrets: Crypt Spirits

My most recently GURPS game, "Overgrown Secrets", revolved around a single monster type: the crypt spirit. Mysterious Glowing undead with a voracious appetite for souls. We had fun short game, and I'm sharing this monster now so you can use it. I'm hoping to share more "Overgrown Secrets" material so you run the game with minimal preparation. But the core of the game was the crypt spirits, and I'm sharing them first.

Crypt spirits are humans who have preserved their soul using a magical metal apparatus whose secrets have been lost to time. They can survive indefinitely in this apparatus, but can do very little while inside. In order to act they must take over someone else's body and burn their host's soul as fuel. While possessing that body, they have great power, but they will need to consume a new body and soul within 40 days. Thus they are driven by their constant need for hosts to be anything more than a powerless consciousness in a crypt.