Great Old One

The Great Old One was worshipped by the ancient Illithid, its depiction can be found in all of their temples. As far as the wizards and clerics can tell it was not a deity (at least not like the others), but an alien and unknowable being of immense power. It supposedly helped the Illithids establish their inter-planar empire and subjugate most other species to rule them as slaves. It disappeared not long before the Illithid civilization fell apart. It had been "slumbering" for millennia and the prophecies of the Illithid foretold that it will awaken "when the stars align" and the "faithful make the ultimate sacrifice". After learning these the Archmistress of Magic, one of the Living Gods ruling the Imperium, ordered all of its old temples purged and then sealed. The seal of the Archmistress was supposed to keep these places buried until the End of Time, but with the destruction of the Living Gods the seal weakened and eventually diminished, allowing people to enter the complexes again.

In the meantime the stars once again aligned in the prophesized formation and the call of the slumbering Great Old One became strong enough to bend the will of Lord Ryhel who organized an expedition to explore one of these complexes with the help of the Avalanche.

But his call was heard by others, the seelie fey of the Summer Court feared the return of the Great Old One so Prince Sagrell, one of the immortal leaders of the fey, sought to organize a group to prevent his return, the Children of Light, to whom he appeared as the Bright Lord.

But the Avalanche thwarted their plans and Lord Ryhel reached the altar in the Inner Sanctum where his death (at the hand of the Avalanche) served as the necessary sacrifice to awaken the Great Old One. Furious in defeat, Prince Sagrell brought the entire complex down, crushing the Avalanche and all others who were in it (fortunately Rhelora later pulled them out and had them resurrected to continue fulfilling their contract with her).

Now the Great Old One is awakened and it is unclear what will happen. Most clerics quickly assert the he is a deity of great evil, but most scholars agree that his nature is so alien and beyond our understanding that neither the "good" or "evil" labels have any meaning. No one knows what he wants and what will happen, but one thing is certain: the world will be changed forever.