The Tau are about combination and ranged attacks. They favour plasma weapons and railguns; whittling down the enemy from afar. The subservient races that join forces with the Tau fill any gaps in their armoury.
The Kroot are the skirmishers - able to close right in on an army and rip them apart. The Kroot can cannibalise the dead, providing a permanent health boost for the army. Those players who favour quick raids and guerrilla ambushes should find the Tau the perfect partners.
Rather than offering a traditional series of missions, the developer has adopting a 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.
Above: The Kroot ride lesser beasts (with huge teeth) in to battle
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 this "the best expansion pack, 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.