First mentioned in The Colour of Magic, these supernatural entities (also known as the "Old High Ones" and the "Ancient Ones") are more powerful than even the gods. In fact they gaze upon infinite pantheons of gods across many worlds in many universes.
It is suggested that while the universe causes mere people to observe it, the Old High Ones observe, and therefore allow the universe to continue.
They have in the remote past exercised their power to prevent the Disc from being utterly destroyed during the first Mage Wars, shortly after the creation of the human race and created the Law of Conservation of Reality. Under their tutelage, the Gods of the Discworld were forced to acknowledge a higher authority, and the human race was recreated a good deal smaller and less powerful, so as to prevent them from successfully challenging the Gods again.
In The Light Fantastic, they are mentioned as having smelled all the cities of all the universes (including Calcutta, and Marsport), but finding that "these fine examples of nasal poetry are mere limericks" when set up against the glorious odor of Ankh-Morpork- a smell so powerful that a statue was put up in its honor, commemorating the time when an invasion attempt by a neighboring nation was thwarted when the attackers' nose plugs gave out.
Azrael is the only one of the Olden Ones named and acknowledged in the Discworld canon.
It is possible that the names mentioned in the Rite of AshkEnte in The Light Fantastic where Old High Ones, as they appeared to control Death, and as Death is above the Gods, they are unlikely to be Gods. They are:
The Old Ones or similar feature in many different fantasy stories from Lovecraft, Stephen King's Dark Tower and Anne Rice's Vampire Chronicles. In fact even in Lovecraft it is not all that clear who Old Ones are. Sometimes they are seen as creatures before humans (perhaps like the giants in the Bible?), later they are linked to Yog-Sothoth and then there are thought to be the residents of K'n-yan. Yob Soddoth could be Discworld version of him?