Umrans are evil spirits, too. Umrans rise from their graves by themselves, not like skeletons. These are the people who died a violent death and then were buried wrong way or weren't buried at all. Every full moon there's a probability of that the corpse will rise from his grave and will become an umran, but this probability decreases with full body decay. The pathfinders see how the blue and green umrans lose their heads time to time literally - their heads just fall off the bodies.