Mankind was trapped beneath the Shield for thousands of years. Five years ago, this changed; they broke through. Using their plasm, they shattered it.
The fragments rained down upon the world city, destroying nearly everything.
The fragments are magic. They are the apple in the eyes of the warlords whose powers are based upon the magic they wield, the plasm they generate, and the people they rule.
Merchant explorers seek out the skeletons of the city that was destroyed, seeking plasm, shield fragments, and human life to sell to the warlords.
Plasm has been changing people. They are transformed when they are exposed to an incredibly high concentration of it.
Concentrations that were unknown till the last days of the utopia-like world city, they are lacking from the archaic basis that defines what plasm does. They are the part of the system that nobody could predict.

Plasm carries advantages that cannot be ignored; It is human will embodied. It shapes the world to the desires of its wielders, but it shapes its wielders to resemble its source.
Now what could that source be?