Nightside
Dead Boy traded away his original name as part of his deal.
His name obviously wasn't always Dead Boy, but that's all anyone ever calls him and no one acknowledges that he was ever called anything else. He may not even remember what his original name was.
Dead Boy made his deal with Lilith.
Dead boy never says who he made his deal with, saying something along the lines of worse than the devil and Lilith was hanging around the Nightside for the past 30 or so years in disguise.Any coincidence that when dead-boy made his deal is around the time John was born (John's ~30 years old).- Technically, he never said that he'd made a deal with something worse than the Devil, only that the Devil would've given him a better deal.
- There's a short story which details Dead Boy's origins and Josses this theory.
Lilith did something truly torturous to the Detective Inspectre in the Bad Future.
In Hex and the City, John overhears the Bad Future's Larry Oblivion wishing he'd died for real, like his brother Tommy, rather than (un)live to see the Nightside brought to ruin. If Hadleigh Oblivion had also been killed outright, Larry no doubt would've wished he'd died like both his brothers, which rather implies that the eldest Oblivion brother met a fate even worse than Larry's. Lilith's minions probably weren't strong enough to take Hadleigh down by themselves, which means she more than likely defeated him personally. The good angels only repaired the damage their side had inflicted in the Angel War.
Jude suggests that, because they are the good ones, the hosts of the Shimmering Plains would repair the destruction and injuries the Angel War had wrought, although restoring the dead to life was beyond their power. The next few books, however, establish that the angelic invasion did have a lasting impact on property (e.g. the Cavendishes' financial loss from destroyed real estate) and people (as with Count Video needing to have his skin stitched back on). This doesn't necessarily have to be a Canon Discontinuity, if Heaven's angels only repaired the harm they had caused, leaving whatever destruction Hell's agents had wrought for mortals to clean up. John is the Could've Been King
We know that the Travelling Doctor is a regular visitor to the Nightside. We know that there were many horrible things sealed away in the Time War, and it stands to reason that the Nightside is one of the few places they could cross over. Now this one requires a bit of a stretch, but we've seen John grab things from alternate timelines. Is it possible that, in the Bad Future, he became the Could've Been King, and his army of Meanwhiles and Neverweres are things pulled from side timelines? On another note, a Dalek invasion of the Nightside would be a trip and a half...
The man-eating house from Something From The Nightside is a Stable Time Loop.
We know that the house was sent by John's Enemies to try to eliminate him. However, we later learned that the Enemies who sent it probably couldn't have summoned it up from some far-distant dimension, as John assumed, because they were the last survivors of the Bad Future Lilith nearly brought about, barely able to strike at John in the past; had they still been capable of bridging the barriers between realities, you'd think they'd have fled to one rather than hung around in a desolate wasteland trying to change history. We know from the Lilith War that Walker cloned himself another house, which was killed by its intended prey, and that John's Enemies created the Harrowing from human corpses. So why couldn't they have created the house from the first novel out of the mangled remnants of Walker's clone? They send it after John in the past, he kills it with help from Suzie, Cathy and what-little remained of Joanna, and Walker's people collect the samples from which they'll clone the exact same house. John picked Walker's pocket at the party at the end of Judgement Day.
The story "The Big Game" has John using the Portable Timeslip, yet Taylor is also still working as a P.I. and another character refers to Walker as if he's still running the Nightside. John first learned about the Timeslip-pocket watch in Just Another Judgement Day, and yet Walker still has both his job and the watch at the beginning of The Good, The Bad And The Uncanny. As word of Walker's death spreads like lightning between Uncanny and A Hard Day's Knight, with the latter starting out mere moments after the end of the former, the only way that John could be in possession of the watch before he killed Walker and took it is if he stole the thing, and Walker didn't reclaim it from him until after "The Big Game" happened. As Walker was distracted by his medical issues, and still hoped that John would be tempted into accepting his job offer by the perks of owning such a watch, he let it slide for a while, but took his Portable Timeslip back in the short time between "The Big Game" and Uncanny.