To become a high priest, one must undertake years of extra study, generally under another high priest, in addition to serving their community. A high priest will have at most three priests training below them to replace them when they are ready to leave their position. These priests in training may spend years, or possibly even die in the position depending on the high priest they are training under.
Although rare, it is important to note that an individual with a particular connection to a diety can be considered for and offered the position of high priest.
There is only ever one high-priest of an order or particualr religion at a time. In order for one to reicieve this title, which they keep until retirement or death, which ever comes first, they must be a second, third, or fourth priest of their order, have been training under the current high priest for at least ten years, and pass an interview by the council of high-priests.
The interview with the council is often the most difficult part of the process, as if the other high-priests do not believe that the priest is emotionally ready, or that they are capable of fulfilling the duties of their order, then the council can skip the individual and appoint a different priest of that order.
Becoming the high priest is offten accompanied by a small ritual with the passing on of important documents and artifacts of that order from the previous high priest to the next. This ceremony can be a grand and lavish affair, or a small and intimate affair accompanied by only the previous high priest, another high priest from the council and individuals important to the new high priest.