Primordial Combiners
The two nigh-omnipotent entities of primordial existence, the dark cloud and the divine light, found themselves traveling forward in time. The Darkness, along with God, ended up in the short-lived universe that the latter had created.
The first thing that came to the attention of the oldest and most powerful entity in existence was that this universe was dying. Not long after, the next things that came to her attention were two supermassive gestalts consuming everything that they came across.
"What in the name of existence are these monstrosities?" she snapped.
"They," her younger brother drawled, "are the original Leviathans. I created one with feminine characteristics, and the other with masculine ones. However, they are much more than just that. Each of them is a primordial combiner, or gestalt, consisting of a host of significantly lesser monsters. Each of them is as powerful as Michael and Lucifer put together."
For all her power, the Destroyer had to resist a slight tinge of terror.
"Although not as powerful as either of us," reassured the Creator, "the harmonious combination of so many minds resulted in a deadly whole greater than the sum of its parts – times two."
The dark essence was not amused by this description of synergy.
"I created them as a soulless experiment in cosmic balance," revealed her divine companion, "As I created, something had to destroy but on a leash. Just as I created my four immediate sons based on light, I created these two combiners using, ahem, your primordial blood donation – fresh blood that, once shed, became scattered all across existence."
The thought of his older sister's primordial blood of darkness terrified him there and then. To his surprise, she digested the account without emotion.
"However, they tried to reproduce," he continued, "and in so doing, they gorged themselves upon this universe instinctively. They would have done the same to other universes in the works, had I not dispatched all four of my immediate sons and more to stop them."
For her part, the Darkness simply looked forward to observing the unfolding of God's plan against the two supermassive primordial gestalts, the Leviathan combiners.
Author's Note: Not being impressed with how the Leviathans were depicted in the show's seventh season, I decided to introduce the much more terrifying Leviathan of Jewish mythology in my fanfic Bonding Outside The Bar. I have reconciled the two depictions on the basis of being inspired by the combiners in the Transformers franchise (one of a number of established and planned inspirations from that franchise). I stated in the aforementioned fanfic that, "Originally, [God] had created two supermassive and terrifying primordial gestalts, one with masculine characteristics and the other with feminine characteristics. A realization settled in, that if these two reproduced, then they and their offspring would chomp the entire petri dish of primordial and later Creation, as Death had put it. To prevent this, [God] tasked his four immediate sons to banish the gestalt with masculine characteristics to a distant dimension and break up the gestalt with feminine characteristics. This they did, and so the first primordial monsters, the Leviathans, emerged as the unconscious remnants of the latter. More recently, their failed plan to consume the billions of humans was an unconscious attempt to regain the power needed to reconstruct the second gestalt and bridge back the first one, somehow."