See page 145 of Codex: Chaos Space Marines
During deployment, you can set up this unit in blasphemous reserves instead of setting it up on the battlefield. If you do so, then in the Reinforcements step of one of your Movement phases you can set up this unit anywhere that is more than 9" away from any enemy models.
12"
3+
4
2
3
8
While an enemy unit is within 6" of this unit, subtract 1 from the Leadership characteristic of models in that unit.