The bloated corpse of a decaying gigantic creature is necromatically enhanced and fused with an undercarriage for use as a grisly and terrifying airship. The methane gas from tissue decay keeps it buoyant in atmosphere, but it can be even more flammable than hydrogen if it escapes...
When was the last time a character caught a cold or flu? Real people get them every now and again even with solid heating and dry interiors. Your characters are out in the cold and wet, and even inside is not always dry and warm. Imagine having to stop your dungeon crawl, sitting in a tight defensive position, while you all are feverish.