They are a force of meddling. Neither good nor evil, but rather pure annoyance.
Fulton - you might or might not have ntoiced my story. I hope to continue it, but am yet to work a fair amount of things out, so I figured I might as well post a brief description to clear up the basics at least.
The Army of the Grand Design is the present form of a more long-standing cult, that until then was semi-secret and mostly focused on manipulation within the "Iranean Empire". A classical Tzeentchian cult, it had over time attracted much of the old aristocracy, as well as younger nobles, members of an emerging military-bureaucratic elite promoted to aristocracy and, ofcourse, the assorted psykers and heretics that over time flocked towards the cosmopolitan imperial capital of Simonia from elsewhere in the sector. The cult infiltrated all military units and key offices by the time the Iranean Empire began to decline; however, the cult leadership refused to acknowledge this fact and continued to conduct business as usual.
The arrival of the man known only as the Messenger (formerly a rogue Xanthist inquisitor) changed things. Infiltrating the cult and gaining support from amongst the restless lower ranks by demonstrating power and knowledge that was clearly a gift from Tzeentch itself, the Messenger managed to stage a coup, taking over the cult and eradicating most of the old leaders. After this, he gathered up the cult members in Simonia and proclaimed to them that he was sent by Tzeentch to mobilise the cult. Great changes were ahead, and the Master's Grand Design entered a crucial stage, during which a key role was reserved for the cult. The Messenger was sent to rouse it from its slumber and initiate the first step of the Grand Schemer's plan for the Nabataea Sector. After quickly dispatching some remaining dissedents, the Messenger organised another coup d'etat, taking over the Iranean Empire's core regions with little difficulty. Newly-raised Chaos formations, in alliance with the long-corrupted elements of the Iranean military, now formed the Army of the Grand Design, eager to fufill their destiny and reap the rewards that Tzeentch had surely prepared for them.
The second step in the plan is the complete conquest of the Jugurtha Sub-Sector...