Charn is a bizarre problem for the Chapter. For some reason that no one (not even Matthawa) can determine, the spirits of the dead use him as a temporal focus. It sounds flash, but the end result is that wherever Charn goes, he is accompanied by a screaming, swirling morass of psychic energy.
His 'hangers-on' are many and varied. Whilst he has a core crew of groupies, anywhere there is death there are spare souls, and they tend to tag along whilst he is in the vicinity. Needless to say, in the middle of a battlefield this can get rather noisy, and Charn is useless on any form of stealth mission.
Despite all this, though, he is a valuable asset in a fight. The spirits protect him from almost all harm, and strike out at perceived threats who are near him. Charn has learnt to fool them by charging heedlessly into the midst of the enemy, where the spirits do all they can to protect their host, generally by melting the brains of anyone that comes near him.
He just can't get anyone to share a Rhino with him.