Here is the series of maps I made showing Grom's invasion of The Empire! I tried to be faithful to the fluff as far as possible, but as this is sometimes unspecific I have had to guess at where the specific route or areas of operations may have been. Enjoy!
EDIT: Hmm photobucket seems to have distorted map1 for some reason, tried to re-upload it but no difference. :( edit 2: Ah seem to have fixed it, I think it was the resizing.
Map1: Shows the initial invasion route of Grom. He rampages through the Dwarf-held mountains and is joined by the Red Eye Tribes and also Wolf Tribes from the Darklands. He then devastates Stirland, Talabecland and part of Hochland. He then captures the city of Nuln.
Map2: At this point the area referred to as The Great Forest (esssentially Talabecland, Ostermark, Stirland) is already overrun, with the surviving humans trapped inside their walled towns and cities. While Grom rests in Nuln the various goblin tribes spread out to occupy the remaining provinces. Forest Goblins join in as well, striking from Drakwald Forest. However Prince Wilhelm manages to save the Reikland by defeating several individual armies in turn.
Map3: Grom's march to the sea, via Middenheim. Having built a fleet he heads toward Marienburg. The pursuing Imperial fleet engages him and is defeated. Violent storms drive the goblin fleet west.