Great Old Ones

The husks of the first Elder Gods to go Hypernova served 2 functions, one of which was the process of neogenesis that gave rise to the Great Old Ones and continued to power the cosmos
The Great Old Ones were enemy and parasite to the Elder Gods, feasting on their energy
The Great Old Ones typically dwell near the Great Beyond, however during Major Discontinuance Events when they are free to wander the multiverse or if they are freed by means of powerful magic at other times.

War with Atlantis

There are gigantic sigils bursting with equivalent power left on the world by the Elder Gods, Destroyers, Titans and Great Old Ones from early elder days.
So feared were the Great Old Ones, that a cache of artefacts were created by the last inhabitants of Atlantis in preparation for the inevitable final war with the Great Old Ones
Some 250,000 years ago, an allegiance of the Great Old Ones emerged seeking to draw off Hecate’s power. Their army of Titans and other minions attacked Atlantis in a colossal war that saw Atlantis defending itself with uber-level spells, Apocalypse-class artifacts and cosmic beings of their own. Ultimately the war shattered Atlantis leaving behind only the Atlantis core and a broken remnants of its higher levels.
As a result of specific uber-level spells and the detonation of the apocolypse class artefact, the Infinity Conjoiner, all Great Old Ones in existence are now actively repelled by Atlantis. Any Great Old One or Titan that gets remotely close or uses divination related to Atlantis experiences immeasurable pain and can not see or experience Atlantis.
The Great Old Ones involved in the assault of Atlantis were/are:

Recent Activity

The Solar Storm working with the BMG uncovered that the Great Old One Karsus Chella is seeking to arrive on Holocene
Karsus Chella is now served by the Alin, Horned God cults and and the Wraith Accord.

Other records

Rogon Saati, Zax Kali-tor and C'Toth, known to the Tethys Ruin whose names are used to invoke arcane power. Clashed with now forgotten cosmic beings that were called Malistar, Jorvi, Min, Sech Noemenah and Ankala
Mond Gorloth, named in the Nimaian Sagas as the Great Enemy of the Bronze God. Referenced in obscure writings held within the Great University of aline & ancient providence
Nctosa & Nctolhu, entombed within Suprurr since the "fall of Atlantis" although which iteration of Atlantis is not referenced - parts of remnant encyclopedia written by Archmage Zeddicus Zed Zahn
Jatoni Alga Aru, known in modern days only by the magical item bearing its name