Old Ones

"The Old Ones are evil. Not bound by your understanding or geogaphy."

- Dr. Fate about the Old Ones.

The Old Ones are a race of extra-dimensional entities and the main antagonists of the episode "The Terror Beyond" of Justice League cartoon series.

History
The Old Ones are a group of extradimensional creatures that are evil beyond comprehension. Mightiest among the Old Ones was Icthultu. Due to their appearance and power, they were often regarded as gods whenever they crossed dimensions.

So too for the people of Thanagar. The Old Ones fed on the worship and sacrifice, but eventually, the Thanagarians rejected them, and moved on.

The Old Ones were indirectly responsible for the sinking of Atlantis. back when they first attacked Earth in ancient They did not get worship from the Atlanteans, and battle ensued. The Atlanteans were losing ground, until their king, Poseidon, gathered all ambient mystical power around to forge a magical trident. The Old Ones were cast back to their own dimension. The loss of the magic around Atlantis doomed it; it sank beneath the waves.

Icthultu returned once more to plague Atlantis. Now, with help from Doctor Fate, Solomon Grundy and the Justice League, the Aquaman repelled them. As Icthultu was killed, his fellow Old Ones retreated back to their home dimension.

Trivia

 * The Old Ones along with their appearance, powers and even their own collective name, were based on the Great Old Ones from the works author H.P. Lovecraft.