It's a BW map. There are 12 beacons - four in the middle, and 8 around the map. There are 4 teams, two bases in each corner of the map. You start with one hatchery, one creep colony, and one lurker for defense - he can't run far from your buildings, but he'll destroy an army of zerglings quickly so you can't get rushed.
Each creep colony spawns zerglings; sunken colonies cost 300 minerals and give you hydralisks. Spore colonies cost 800 minerals and give you cloaked infested kerrigans (be aware that they uncloak when they run out of energy). You can build more colonies to get more guys.
You get points for having control of the beacons - run your units over them to take control of them. If there are multiple players at a beacon, control is granted to the player with the most units at the beacon - burrowing can be useful early in the game. For every 1500 points you get, you recieve a set of heroes.
You get minerals for kills. Don't forget to upgrade your attack/armor.
After roughly 10 minutes (I'm not sure what the exact conditions are, but it always seem to be close to that amount of time), the condense triggers kick in: if you have 30 zerglings/hydras/kerrigans close to a colony, they are condensed in to one super unit. Zerglings turn in to uber-zerglings, hydras turn in to uber-hydras, and kerrigans turn in to dark templars.
You can mass attack by sending the flying critter to a location and moving your mass attack civililian over the mass attack beacon. This is something I think is really lame, and was not in the original map. Be aware that mass attacking results in unburrowing all of your burrowed units; your heroes will not mass attack.
You win if you get 15,000 points, or if you eliminate every enemy. You lose if your hatchery gets taken out.