The Unspeakable Entity
One of the most primordial Incarnations, he is the embodiment of entropy, the decline of order into disorder. However, in his natural state, his power was so disruptive to creation, he was permanently banished to a dimension that he could not escape from. His destructive effect was so immediate and unwanted, he was exiled before there ever was a name for him. Indeed, very little information about his very existence appears in any record. For eons he has raged in solitude and would rather destroy all of existence, including himself, just to end his isolation and torment.
Although locked away, his power still affects the rest of the Multiverse in a predictable manner to form a certain balance. This also means that he is able to contact other planes and universes when reckless beings take actions that tear into the barriers between universes. In this way cults, demons and other nefarious beings have learned of his existence and are often seduced with promises of power if they could free him. Freeing him has always been a nearly impossible task for his dark dimension is specifically impervious to outside attempts to open it. At best, one could peer into it with extremely powerful effects which would allow him to use that connection to communicate.
The only way for him to ever break out of his prison was for the Singularity to be ruptured since it is what keeps the rules and parameters of his own universe in place. As the embodiment of entropy, of course, he would know how to dismantle it. He just needed to convince someone powerful enough to do his bidding.
Like all other universes, the Rupture of the Singularity broke down the barriers of his confinement. Although drained of his full power from this event, he is now free to vent his ancient rancor and quickly tethers together a compulsory army in a quest to keep the Singularity from being reformed and end the misery of reality.