The purifiers are a race of sentient and self-aware robots created by the protoss.
The protoss began experimenting with technology to create the purifiers, to act as weapons. The Conclave, however, saw them as no more than machines, and treated them as such. In retaliation, the purifiers rebelled and were shut down.
Centuries later, the Khalai attempted to recreate the purifier technology on Glacius. Fenix placed his current memories into a prototype warrior, whom after awakening would believe himself to be Fenix.
During the End War, Artanis and the remaining Daelaam traveled to Glacius to retrieve the purifier technology. They managed to do so before the Tal'darim Death Fleet could destroy it. Afterwards, Fenix suggested that they awaken the original purifiers on Cybros. The purifiers joined the Daelaam under Fenix's leadership, but were given free will.
The Purifier symbol