After The First of the Fallen was created, God realized that he needed to make something to keep The First in check. He therefore created The Second and Third of the Fallen; demons of equal power to The First who would form a three-way standoff for eternity, with none of them daring to seize full control of Hell lest it be destroyed in civil war. While other triumvirates also reined during this time, including one featuring Lucifer, the First, Second and Third of the Fallen held the true balance of power.
After discovering that he was dying of lung cancer, Constantine sold his soul to The Second of the Fallen. However, he also sold it to the First and Third as well. Knowing that they wouldn't risk war in Hell, he then slashed his wrists, forcing them to heal him completely.
The stalemate remained until The First (whose existence predated Hell) found out that the Second and Third were not his equals, but merely extremely powerful demons with major weaknesses. He then killed them both, breaking the stalemate and allowing him to go after Constantine. The remains of the Second and Third were then transformed by a mysterious third party into the Twin Blade.