This is a mechanical flying creature that is attuned to a Rune Priest's brain pattern. It can fly ahead of the Rune Priest so that he can see what it sees and it will protect him if he is attacked. Enemy infiltrators may not set up using their special rules against a Space Wolves army that includes a Rune Priest with a Chooser of the Slain unless they first roll a 4+ on 1D6 (roll separately for each unit). In addition, Rune Priests that have a Chooser of the Slain count as having an additional close combat weapon in close combat (ie, +1 attack), although this modifier does not apply if the Rune Priest already has an additional hand weapon.
Chooser of the Slain
Source: Codex: Space Wolves, p. 15