The first recorded emergence of this gift is among the Malanari. They are known by their obsidian-like eyes that momentarily blind them to all but the sight of the great web. The longer they are in contact with the Web - the longer their eyes remain blackened until eventually they loose the ability to see in the 'normal' world.
When the Malanari made contact with the Tenebril - they noticed that their smaller, Halfling allies were also able to make contact with the Great Web as well and were trained as oracles.
The prophecies of the Obsidian Oracles have been recorded in the Obsidian Texts and are often consulted by the faithful as a means to gain insight about future events.