StarCraft/Zerg mission 9: The Invasion of Aiur

Building the Initial Attack Force
You start out with a sizable base in the upper left corner of the map. Basically, what you have to do is send a Drone to the Khaydarin Crystal Formation (the location of which is shrouded in fog of war but visible nontheless), protect the area around it for 10 minutes, and bring the Drone with the Crystal back. But there's a lot of fighting involved... you don't have to raze any bases, but since about 80% of the map area is controlled by the Protoss, you will have to go through their bases. Zerg strike forces in the campaign usually involve air units, but your base doesn't yield enough gas for that. Which means, of course, that you must expand.

After building your economy and an adequate system of static defense, concentrate on teching up to Mutalisks. You need these guys because the Protoss will use Reavers aggressively in this mission, sometimes airdropping them directly into your base. Sunken Colonies have shorter range than the Reavers, so they're useless, and of course there's pretty much no Zerg ground unit that can stand up to a Reaver for long. So keep a healthy number of Mutalisks near your base at all times to deal with this threat.

Luckily for your gas needs, there's a free resource deposit in the top right corner of the map. To break through the light defenses there are hold the deposit, you won't need any more than a full group of Hydralisks and some Zerglings.