The Ancient Ones were the celestials that had been around seemingly since the beginning. Every celestial knew them and a lot of the foundations, the core principles they believed in, and how the celestials' society was set up worked around them.
They had become far too large of a group, for a celestial not to join them. Whether that be one occurring naturally because a world needed one, or a celestial being invited due to them being worshipped.
For beings to create an organisation like so, where they practically controlled the gods, also meant they had to be terrifying themselves. In the past of nearly any organisation, empire, or group, the hardest task was controlling them all as they grew in size. This was the downfall of many great empires that fell.
