Shadow Ascendant

The Forgotten Shadow is a powerful religion that sweeps through the Forsaken. A devotee may throw off the shackles of their zombie-like body, becoming one with the Shadow. Those who succeed become shadow ascendants. Outsiders, however, do not understand the process that fuels the ascendant's evolution from a physically powerful (but slow) corpse to a quick and deadly shadow. The Cult of Forgotten Shadow, however, preaches that all Forsaken are born from the Shadow, and that they carry a portion of the Shadow within them. By strengthening their ties to death and undeath, the ascendant manifests their inner Shadow, bonding with it until they become a living shadow: the ultimate manifestation of the Shadow from which they were supposedly born.

For more information, see:

http://www.wowwiki.com/Shadow_ascendant