The group was a secret coven of 13 people gathered together by a man named Slayton. Slayton fancied himself a warlock, and during one of his rituals, he stumbled across what he believed to be a hole in the balance of power between Heaven and Hell.
He came to believe that this hole would be filled by something given enough time, as nature abhors a vacuum. To speed things along, he created The Group. Each of the 13 people agreed to consume one another until there would be only one person remaining. This final person would then be the vessel through which a beast would come to fill the void, a beast he called Urizen.
Before the ritual could be completed, however, Spawn intervened and poisoned one of their fellow "meals."