'Oracle' is a title honoured to one being proven exceptionally wise and far-sighted.
The Oracle is also called 'summoner' or 'watcher'.
This might come from its power to summon council members at any time and from any place, as well as the guardians, which also are chosen by the Oracle and assigned missions by it.
'Watcher' comes from the many different ways of catching glimpses of the past, present and future, as well as their many variations through meditation or different kinds of mirrors.
An Oracle observes the many worlds but is not allowed to take action of consequence unless the Council of Brothers has agreed to it through vote. It is this rule
that gave birth to the guardians who serve as the active part of the fortress.
The Oracle also has to read and interpret the transfiguration cards, which are the speakers of the elements and is responsible for the fortress and the balance
of all worlds.
If one oracle dies, is removed due to power-abuse or other delinquencies, or decides to give up the title of Oracle, a new one has to be elected.
Anyone from the Council of Brothers can become a candidate, however in the end the vote decides. One by one every member of the Congregation, including rulers of the many worlds,
steps forward and puts a hand onto a stone pole before the candidates. The pole glows in response and counts the vote as being made.
The decision of the vote is represented by a single butterfly that lands on the hand or face of the new Oracle and turns into the mark of illumination.
To accept the title of Oracle the candidate first has to relinquish his or her former name.
After that many other ceremonies follow, most of them cleaning or meditation ceremonies of some sort until the chosen one receives the full power and duty of an Oracle. Only after the final initiation ceremony can the new Oracle reshape the Fortress.
On rare occasions the former Oracle can chose its successor. This is how Yan Lin became oracle in the end. Himerish presented her the Shawl of Power and with it, started the power transmission.
Similar to the keepers of the Heart or guardians in general, not much is known of the Oracles before Himerish.
From what the comics tell, N'ghala - one of the five nymphs - was the very first Oracle.
After her we don't know who came. The picture above shows many faces of Oracles but we do not know their names or if these were all the Oracles or if there were more. Since the Fortress is timeless it is unclear how many years an Oracle serves.
At some point Himerish was chosen and became the Oracle. He served for many years it seems, until he broke the rules of interference and was trialed and banished. In his stead Endarno (actually Phobos in disguise) was supposed to become the next Oracle, but the final initiation ritual was never performed. Again, Himerish was supposed to take on the task of Oracle but this time he decided to introduce the Trinity rule. He chose Yan Lin and Endarno to be his Co-Oracles, you could say. His former banishment however has changed Himerish and he soon realizes he no longer desires to be Oracle but a normal man. By presenting the Shawl of power to Yan Lin he names her as his successor and she becomes the new Oracle.