After the Second Fall of the Angels, Satan and his band of followers were cast into the Pit, where they waged a thousand year war with Pluto's followers. Satan was eventually victorious and declared Hell his kingdom. The remaining Watchers from the First Fall of the Angels were enslaved and made to build Pandemonium. Pluto was the only Watcher to escape.
After Hellboy murdered Satan in his sleep, the death of the King of Hell precipitated a slave rebellion that nearly killed a majority of the demonic hierarchy.
The surviving demon lords rallied to Beelzebub's castle at the edge of Hell. Worried for their safety and the diminish fire in Hell, Beelzebub proposed to call forth Pluto so he may take Satan's place to re-establish order.
The plan to locate the former king of Hell was never carried out as the demon lords were all massacre by a combined onslaught from Hellboy and a mob of vengeful slaves.