Heart of the Swarm Campaign- Zeras: The Crucible

This mission provides the single largest reward found anywhere in the game- not only will completion give you access tot he swarm host unit and completing the bonus objectives give Kerrigan a level, but merely completing the mission will grant Sarah the addition of a whopping ten levels.  Of course, this is as much to reflect the key nature of the mission as well: Kerrigan seeks to harness the might of the primal Zerg by going to the place of their origin.  This trip threatens to result in her destruction, but even success promises to transform her very nature.

At the opening of the mission, you are shown the pool that Kerrigan needs to reach, and then given control of her- and five swarm hosts.  These units, developed by Abathur from primal Zerg DNA, can burrow into the ground to spawn locusts that will quickly hatch and then go to and attack nearby foes.

Almost immediately, you are warned of incoming primal Zerg.  Abathur will indicate that you need to have the swarm hosts burrow into the ground, and you will be given a location on your map- the enemies will not appear and move in until the swarm hosts are burrowed into that place.  The assault itself is small and easily destroyed- you then need to bring Kerrigan to the East, towards the pool.  A short cutscene ensues, with Kerrigan turning protection of herself and the hive cluster newly established to the North over to Zagara.

With the use of the original spawning pool, you have vision across nearly all of the map- the only exceptions are a few areas that units cannot go to, and an area to the South end of the map which is only accessible through two clefts in the plateau there.  You will start out with several colonies and swarm hosts in play for you to use, though you'll need to set to work immediately on increasing your resource stockpiles.

Before long, remnants of Brakk's brood approach from the West- you'll need to move out your swarm hosts and spine crawlers to the entrance you used to reach the pool.  The attack is rebuffed relatively easily, but followed in short order by a larger, more dangerous force from a new brood to the East.