Brood mother Na'fash had traveled here to create stronger claims. Sarah Kerrigan and a small number of zerglings landed on the moon (via sac) in order to reclaim the Na'fash Brood. She discovered feral roaches which she brought onto her side, along with yeti.
The moon was constantly struck by cold windstorms, which would freeze the zerg and protoss in place and force the local protoss base to raise its shields and go into a dormant state, but had no effect on the yeti. As a result, Kerrigan wished to take DNA from a pair of yeti matriarchs.
The zerg encountered Na'fash's corpse... the protoss had got to her first. The high templar in charge, upon noting the presence of Kerrigan, attempted to warn Shakuras. The protoss remarked on Kerrigan's weakened state. Unable to reach the far-off Dark Templar homeworld with the Khala, he instead used psi-link spires. Kerrigan tasked her forces to destroy these spires before they could be activated. The protoss could do so in 15 minutes. If the player collects the yeti DNA, they can take advantage of the cold weather by attacking when the protoss are frozen, bypassing defenders.
Kerrigan destroyed the spires, at the the three protoss bases prompting the protoss to retreat. Kerrigan then ordered her forces to finish off the protoss before they could travel directly to Shakuras and get their warning off.