I'm guessing the Ghost Zeppelin? Well to start, look at what you know about the situation: There was a distress call coming from a Zeppelin. The Zeppelin could not be identified through any Mesh Means. Clearly there was something there that was a brutal efficient killer that could not be seen. The "Killer" was able to get from one ship to the next without going through an airlock with a body. The only way that someone survived was by taking an emergency balloon. This all occurred within 3 hours. So these are some of your key points to start with. Now the survivor was on a passenger ship, so whatever it was probably was taking time to work through passengers. With a limited party you can probably expect this "killer" to work quickly to hunt down anything that enters its territory. It doesn't have to be just one thing either. You could have multiple "killers" working in together to accomplish this. Now you have to ask yourself why? Why is this thing here? Did someone plant it here? Did someone catch hold of a strange transmission and suddenly Exsurgents, Exsurgents everywhere? Is another company trying to run the this Zeppelin company out of business? Basically right now you have part of the method of the killer. You would need to work on developing the motive as well, but as always I'm just a silly rambler.