After a while however the souls became too many as the humans spread across the Earth, forcing Death to take more and more of them each year. Seeing that the Underworld would soon be overflowing, Death used much of its original power to create a second holding realm, naming this one 'Heaven'. The new realm was somewhat smaller than the first, but it was still able to accommodate a great many souls. In spite of this, Death found itself with yet more problems, namely the administration of the realms. The entity now had to take souls from Earth and send them to the two other realms, all while leaving a portion of itself in each realm in order to ensure stability.
Several generations passed before Death finally decided that it needed help. After sending out a call across all the known dimensions, Death succeeded in bringing a god-like being to Earth. Charging this newcomer with the administration of Heaven, Death turned its attention to the administration of the Underworld. Before he had a chance to act however, he was informed that a war had broken out in Heaven. Upon his arrival, Death discovered that the armies of the God being (commonly known as Angels) had begun fighting amongst themselves. Lucifer, the leader of the Angelic rebels (popularly known as 'The Rebel Alliance') met Death at the gates of Heaven and attempted to explain the situation to him. It soon became apparent that God's inept baking skills had driven the Archangel to the edge of insanity, prompting him to rebel. All Lucifer had ever wanted was a plate of fresh scones, but God always burned the damned things. Naturally Death found all of this rather humorous, but saw an opportunity to kill two birds with one scone (ahem). He promised Lucifer an everlasting supply of scones if he'd agree to leave Heaven and administer the Underworld for him. Lucifer agreed, on the fairly unusual condition that he be allowed to rename the Underworld 'Hell'. Death agreed to this, albeit it reluctantly. Lucifer's 'fall' to Hell brought an end to the war in Heaven, though his fellow Angels never forgave him for what they perceived to be a betrayal. This mattered little to Lucifer however, who swiftly consolidated his power in Hell, appointing his close friend Samael the 1st 'Satan' and overall governor.
Centuries came and went, empires of man rose and fell, providing Heaven and Hell with a constant supply of fresh souls. New religions altered the perceptions of humanity, causing the holding realms to alter with them. Hell became a place of torture and evil, Heaven a place of peace and goodness. Well, that's the Christian take on it anyway... In more recent times things have taken a rather drastic turn, though for good or ill no-one can truly say. The once powerful entity, Death, has been dethroned and replaced by a slightly insane young woman with a love of dogs and a penchant for flicking people in the head. Now able to wield the full power of the former great being, Phillippa Morgana makes an interesting replacement.
The situation is much the same in Hell. Though Lucifer still reigns supreme, the position of 'Satan' has been taken over many times by various persons. Of late the post is held by Titus Varus (self-proclaimed 'El Diabolico'), an individual who constantly covets Lucifer's tasty treats.
The only thing that hasn't changed is God...She's still burning the scones...