A great location - though given the size/quantity of the equipment down there, there would have to be another entrance - a cargo elevator that perhaps leads to a mom n' pop garage a block or so away. That way you could have a vehicle lift and garage.
This would also be where the debris from all the underground escalation could be extracted.
Apparently I have logistics on the brain for some reason :)
Not bad, the secret certainly helped it. I have a some suggestions:
"However, usually around 1 in 10 people can make it out, as they're familiar with the terrain. "
To this, or something similar:
"Occasionally some exiles survive, either by blind luck or exceptional skills at woodcraft."
"Whatever the reason, John soon discovered a necromancy spell, as well as sorcery, that can help with one's public speaking, and your talent for it."
Such magics are not normally considered the area of necromancy, but perhaps sorcery. It feels forced - you needed him to have exceptional public speaking, and he uses necromancy, so poof - charisma enhancing spell.
I'd say instead he became possessed by some spirit during his dabbling that wanted to use him for some evil end. It helped him in his quest, grooming him for bigger things. In the end, perhaps John figured it out, and killed himself, or he began to disobey the spirit and the spirit killed him. The tomb may in fact still harbour this evil spirit.
Quite a mixed bag of characters here. Some are rather dull, but they generally improved as their numbers moved up. They felt kind of safe, without a lot of the truly dark edges that the Cosmic Era. That said, there are some that certainly could be the keystone for an adventure, or a significant element.