The High Priest was a Dalek that landed on the planet Florana after its ship was damaged in battle, causing its space-time core to rupture and sending it back it back in time as well. Upon arriving, it convinced the locals from a nearby city that it was an emissary of God who sought to bring enlightenment to the people of Florana. Within a few weeks, the people of the city made decades worth of technological advancements under the guidance of the High Priest, as the Dalek came to be known. During this time, the High Priest also had a "throne" constructed to contain its body, as its Dalek casing was severely damaged in the crash. While outside the influence of its Dalek casing, it developed a more individualistic personality than would normally be seen in a Dalek.
During the next ten years, the High Priest attempted to create a master race among the Floranians. The strongest and most intelligent Floranians were conditioned to be obedient and trained to be soldiers and tacticians, while the less intelligent ones were forced into slavery. Anyone too weak or too old to be of any use was executed in accordance with the High Priest's forced eugenics program. Building a dome over the city and fortifying the city militarily, the High Priest ordered that all other cities be subjugated under their rule. It hoped that the resulting war would further encourage technological development until the natives were advanced enough to create a time ship for him to return to its own time. As a contingency, the High Priest also ordered the construction of a beacon to signal any Daleks present within the time zone.
The Fifth Doctor eventually landed on Florana ten years after the High Priest's arrival, his arrival in the time zone being a result of the TARDIS being hit by the blowback from the ruptured core during the crash ten years earlier. Although the High Priest didn't immediately recognize the Doctor and the TARDIS, it was able to determine his identity by using the data banks in its damaged casing. Now with a means to leave Florana, the High Priest appealed to the Doctor to use his TARDIS to take the Dalek away from Florana and return it to the Dalek fleet. Although the Doctor was initially displeased with the idea of helping a Dalek to return to its life of wreaking havoc across the universe, he acquiesced upon realising that removing the High Priest from its seat of power and from Florana itself would ultimately be best for the planet and its people. However, before it is able to leave, Thane, the head of the church, discovered the High Priest's deceit and declared it a false prophet. As the crazed priest brutally murdered the Dalek, it expressed pride in how Dalek-like Thane had become. (AUDIO: The Elite)