Rather than offering a traditional series of missions, Dark Crusade will employ Risk-style meta-game. Between levels, you'll get to choose which territory to attack and with which army. Wresting control of most territories invokes a simple skirmish battle, with each territory providing a bonus ability. You might take a detour to attack one area that adds a Dreadnought to your starting army or battle for a space-port that allows you to drop into any territory, not just those adjacent to your borders. When it comes to taking a stronghold - the headquarters of your opponent's territory - a specific scripted mission will launch; you'll play a mini-story based on your invasion.
So is Dark Crusade "the best expansion pack for any game, ever"? It's certainly incredibly generous: two new armies, masses of upgrades, and a whole new way of playing. We'll have the full verdict for you soon.