Wights are a race recently created by an unknown being. It is known, however, that Wights are the remnants of Humans with a certain disease.
The disease is called Minor Necrosis. Despite the name, the body does not die. Instead, the body is reanimated the moment it dies. As soon as it's reanimated, it can't be called Human anymore. It will not recognize people it once knew. In fact, none of it's former memories will exist. It can, however, remember how to use tools, and even create new memories. The Wight then has the ability to spread Minor Necrosis through claws or teeth. Anything the Wight touches then has the ability to spread the disease. So, any weapon or tool it wields can infect.
Wights differ from Humans in that they cannot feel pain, but they can die. Because of this, Magic is of little use against them. The most effective way to kill them is through decapitation. They also cannot bleed, so removing their limbs is useless, unless you need to slow them down.
Wights are considered Undead, so Light magic is very effective in killing them. They're also terrified of light, so they dwell in places where light is limited or nonexistent.