The restless ghost is the most common thing to rise from a fresh grave. It is not malicious so much as lost - a deep-blue, drifting shape that wanders the lanes looking for the life it left behind.
Left alone it will press toward your homes and frighten the people sleeping there. A resident scared badly enough can sicken, and the next morning you may find a new grave - which, in turn, can rise. That is how a single bad night becomes a spiral.
TIP A well-lit street turns a restless ghost long before it reaches a door. Lanterns are your first and cheapest defence.