Deathwalkers are humans who have been marked by the restless.
Wraiths will choose humans to teach about the Wraith world before they die. This will sometimes include arranging a violent death for them to increase the chances of a wraith result. An educated enfant prepared to serve is a very valuable commodity.
There are also those mortals who have been touched by fate....they already know how they are going to die. Wraiths look for these people because they are almost guaranteed to join the Legion of Fate once their "incubators" expire...or they are at least entertainingly crazy.