Cavir is a planet populated by feral zerg.
Daggoth used the planet as a birthing colony for his Tiamat Brood. After the death of their master, the zerg became feral and fought each other. During the Second Great War, Abathur discovered a small group of lurkers among the survivors.
Sarah Kerrigan was able to control the group of lurkers, but couldn't morph more of them. Abathur informed her that a key sequence was missing, and that a lurker den was required. One was found nearby and the group of lurkers was used to protect it from more feral zerg until Broodmother Kilysa arrived to recover its essence.