Atum killing and slaying hundreds of elder gods/Demons all at once
Atum is son of the female Elder God known as Gaea or Mother Earth and the Demiurge, which is the sentient life-force of the Earth’s biosphere.
Mother Earth dispersed her own immortal energies into the planet that bears her name. When Mother Earth / Gaea gave birth to Atum he began a campaign to destroy all the Elder Gods who had fallen from their original divine nature and turned into monstrous demons. These murderous demonic entities were waging cannibalistic wars against each other to try and gain the most power. Atum's war against the demons ended their chaotic age but in the process Atum became the Demogorge, a fairly monstrous entity all on its own.
When the elder snake god Set achieved the power to manifest on earth again, the god eater was asked by Thor to help. Eventually Atum appeared and assaulted Set until the serpent's power was exhausted.
When the Greek Titan king Cronus launched an assault on Olympus once again, Atum observed and warned the Olympians that if Cronus won, the Greek gods would all be devoured by Demogorge.
Atum resides in the sun and is completely immune to the heat, pressure and cosmic forces of the sun. He can fly at light-speed, generate intense light and heat. He can also absorb/consume foes thereby gaining all of their abilities. Atum is by a considerable margin one of the most powerful entities of the whole Marvel Universe.
From time to time fragments of Atum's energy as the Demogorge have been stolen.
Cthon hived off a tiny portion of the energy and locked it within a page of the sinister tome known as the Book of the Darkhold.