Castor's raiders bring in additional intelligence regarding the Spinward Front/Periphery:

The main Eldar faction operating in the area is known as the Children of Thorns Kabal. They are a bunch of savage and vile outcasts - so much so that they are exiled from Commorragh and shunned by other 'Dark' Eldar. Which is no mean feat, given the nature of the citizens of Commorragh!

E'danas, Castor's pet Eldar reaver captain advises him to stay away from Kabal. They are too factitious and too erratic to be of direct use to the Apostle.