Shadow Master
Codex Supplement: Raven Guard, p. 4
A unit with this special rule may Set to Disappear instead of Setting to Overwatch. At the end of your opponent's Charge Phase, a unit that has Set to disappear may be removed from the battlefield and placed in Reserve. The unit may be removed even if enemy units are within 3" of it. At the end of your next Movement Phase, you may place the unit anywhere on the Battlefield more than 9" away from any enemy models.