A Great One is a superior being, above all levels of existence a human could even hope to achieve. Merely learning the eldritch truth around these great beasts could drive a man mad, so that his mind could be used to help another begin to learn that same truth.
Insight is a measure of knowledge of the unknown. The more you know of this, the more you can hear, even see things that weren't normally there.
Creatures seems to gain a voice, babies begin to cry in the abyss, and lord forbid you pass a certain point, creatures like the Amygdala become visible.
You are a Great One, more specifically, you are the Moon. You were rebirthed in this state after slaying Flora, the one previously in this position.
Now, you watched the years-no, the millennia pass, as you grew, clueless of the world outside, and growing more and more. It seemed you had been forgotten by the humans you were once one of.
Then, one day.... somebody prayed to you