Skinwalkersare Shapeshifter that possesses the ability to shift into all animal forms, including humans. According to the legend of the Navajo tribe, Skinwalkers are evil witches whose powers allow them to transform into any animal on earth. This includes other people.
A shapeshifter must kill another shapeshifter from their own family in order to become a skinwalker. Luna's mother died in childbirth when she was born. This gave Luna her skinwalking powers.
Tommy Mickens has also become a Skinwalker by killing both of his parents, one of whom was a shapeshifter. He later unexpectedly/unintentionally shifted into Sam Merlotte and thereby caused some trouble for Sam. ("I Wish I Was the Moon")
If the skinwalker stays shifted as a human for to long, they will experience some side effects. These include falling unconscious, running a high fever and vomiting blood.