The Old Gods are creatures of unfathomable horror that dwell in Nullspace. Also called 'Great Old Ones', or 'The Formless', 'The Many-Angled', they are creatures of such puissance and unchecked power that their mere existence warps reality around them.
They are hideous and defy any semblance of natural order or evolution. Some are capable of inspiring insanity simply by being observed. These tortured beings are the echoes and corpses of the First Gods who were destroyed when Oa broke the First Firmament, and found themselves displaced into this nightmare realm.
Younger, hungrier Old Ones periodically make their presence known at the edge of our reality. They peek into the rips and tears between the universe, reaching out to feed and snack. Sometimes they gain enough of a psychic foothold to project their will, allowing cults of devout worshipers to call them forth. They may send emissaries and agents, often called Outsiders, to advance their agendas. Their motivations and desires are unfathomable, beyond such simple definitions as 'hate' and 'cruelty'. Some hunger for souls, while others feed on the psychic energy of violence. Others inspire madness and feed on disorder. If the Old Gods have a single weakness, it is that they are as blind and confused in our universe as we are in theirs. The light from our universe is darkness to them. It requires specific effort to open the Outer Gates to Nullspace, let alone to wake an Old One and then guide it to the rift.
Old Ones do not just threaten worlds, or life. They threaten reality itself, seeking to undo creation. Some, such as Trigon, are born of mortal malevolence given life in the Negative Zone. Dormammu hungers for the Life Entity at the heart of Earth, seeking to corrupt all life in the universe through it. Still others, such as mighty Azatoth and Yog-Sothoth, are so hideously alien and repugnant that they actively damage our universe simply by venturing near it. Fortunately, many of the Old Ones live in a state of eternal, endless dreaming, rarely waking. The Endless seem to play a role in this, particularly the entity known as Dream.
The least of the Old Gods is a threat of incalculable magnitude. Deities of Earth have been slain in the effort to contain such an invasion, beings that are otherwise nearly immortal. The power of an Old God can consume entire demesnes, stripping it of power and souls alike.
At the fall of Atlantis, desperate cultists reached out and broke open the Outer Gates to try and prevent Ragnarok. They succeeded in calling forth the ancient alien entity C'thulu, though Merlyn and the Vishanti bound him to the sunken city and banished both into a demiplane that sits at the bottom of the Pacific Ocean. There he slumbers, the Atlantean city of R'lyeh twisting around his eldritch horror.
In 1908 CE, insane zealots of Hastur summoned its emissary: Irktuni, Knight of the Yellow Banner. Their ritual drew the Old One to a region of Russia called Tunguska. The resulting battle with the Slavic deities resulted in the death of the Primarch Deivos and many of his children, their dimension drained of energy and the souls within consumed. Surviving gods dwelt on Earth as mortals or found other realms to inhabit.
Should one of the sleeping Great Old Ones actively be summoned into reality, the response would be cataclysmic.