The adostra are a strain created by the zerg using xel'naga essence mixed with an unknown species capable of housing it. They were created by Abathur and Overqueen Zagara years after the End War. Their name means "dreamer" in the xel'naga language.
Before she ascended to become a xel'naga, Sarah Kerrigan informed Zagara of a peaceful, not fully sentient species that could house the genetic essence of a xel'naga. Zagara tracked down these species and had Abathur modify some of them with xel'naga essence recovered from Ulnar, creating the adostra. The adostra's psionic abilities were beyond even the protoss, and reached down to the cellular level to coax planet life to grow. The adostra were kept in eternal slumber in growthing pods to achieve this, and to transport the adostra, Zagara had Abathur create a new aerial breed of zerg named the skyrling. Within a short time their abilities seeded abundant plant life across the planet Gystt, once burned by the protoss and devoid of life.
Zagara intended for the adostra to spearhead the new path she had chosen for the future of the Swarm; the cultivation of life across the galaxy. However, the revitalization of Gystt attracted the attention of the protoss, who suspected Zagara's motives. When Hierarch Artanis came to Gystt with an attack fleet, Zagara contacted Valerian Mengsk for aid. On Gystt she told them of the adostra and how they were created, and of her intentions for them to not only seed life on other worlds, but to have them create new sources of food for the people of the Terran Dominion and also aid the Daelaam however they could.
Unknown to Zagara, Abathur protested the idea of a truce and had created a second species with the xel'naga essence they recovered, the chitha. Abathur planted the chitha in the hives where Zagara was keeping the adostra, and intended to instigate another war by having them attack the survey teams sent to investigate the sites; Abathur also intended the adostra to be destroyed by the protoss and terrans, considering them abominations.
Through the efforts of the three races, Abathur's machinations were thwarted and at least some of the adostra were saved.
The adostra are kept in eternal slumber in zerg pods. In their dreams their latent psionic power as xel'naga calls out to life wherever it lay, generating accelerated growth. This made the formerly barren Gystt into a lush jungle. Zagara indicated their powers could be directed to grow specific plant species. While only plants were shown to have been created, Zagara indicates the adostra may be able to cultivate non-plant species to grow as well. In addition to stimulating growth, adostra can cause plants to wither and weaken, allowing them to shape plant-based structures to be stronger on one side and weaker on another.
According to Zagara, the adostra are entirely non-violent and sleep forever.
To respect Kerrigan's wishes that the zerg never assimilate the essence of the xel'naga, and because xel'naga essence cannot be replicated by the Swarm, the adostra lie outside Zagara's direct control; they have no connection to the hive mind and cannot be bred by the zerg. Creation of more of them would demand the acquisition of more of their original species to be modified, as well as more xel'naga essence.