In early days, at the end of the battle of the valley of sighs, the gods of dark, light, grey, and green met at the top of Mt. Doom. There they agreed not to manifest physically/directly – Proxy would be the way of battles – this was modelled on the way of the Celestine Powers on Helca. NO more direct confrontation, no more direct interference. Little else is known of this, other than it has governed their interactions since. The accord allows for the deities to speak directly to their followers or against their enemies, but nothing else.
The power of a divine Principle can be invoked by Connexion, to make a pronouncement of Weal or Woe, a Blessing or Onus, vision/dream revelation, an Observance of the sojourn or even a pronunciation of Agenthood in extreme circumstances.
d20 [specific] Requirements:
A prayer of warding must be uttered here since the god has directly spoken, it can be fought against by devout. Speaking can cause onus and curse. And triggers for augury. It can be countered using a prayer with Connexion. Pronunciation of weal or woe or vision/dream revelation.
Mt. Doom was given its name at the time the accord was enacted.
Whenever the gods clashed directly, some terrible natural event happened elsewhere in the world. There would be nothing left of the world for the victor but a ruin. And so, the came together to form a pact, an accord between them.
The Celestials were not present when it was made, and have never agreed to abide by the agreement.