This is based off of Preserving Order In Post-Apocalypse Labyrinths (As shelters!), I'm trying to focus on and cover each point in more detail to gain additional insight here. In this case, it's Point 1.
Premise: After the discovery of eldritch horrors and the existence of magic (never mind the subsequent apocalypse), people went underground. Special shelters called Labyrinths channel the chaotic energy of the one eldritch horror the humans managed to slay into powerful magic that protects them from discovery and supplies the shelter’s energy needs. (Think Cthulhu equals clean energy =) ) However, the Labyrinth has some interesting effects, only one of which we'll cover here:
Point 1: Anyone living inside is biologically immortal; they don't die from old age, poison, or disease (including cancer) and regenerate all but the most extreme damage (think axolotl) but they can die from starvation, dehydration, and oxygen deprivation. Basically, people age until they are at their physical prime, and then their aging stops right there.
There are fifteen Labyrinths, distributed all over the world, with two million per each Labyrinth. As part of a poorly planned movement to unite humanity as a group, the survivors were distributed between different Labyrinths, causing these Labyrinths to have a multicultural population. This may cause conflict; the survivors were distributed randomly, and the politicians (who want to keep power and rule over the survivors) and the wealthy (who invested money and want to claim power) will conflict with the poorer people (who will likely despise the expenses and corruption of the wealthy and elite and fight against their attempts to control them).
As for overpopulation, the first generation will turn to crystal right after the next generation reaches 25. (In this case, a generation is a group of people born and living contemporarily.) This sudden, strange, and quite possibly horrifying transformation will have a psychological and societal impact; how would people feasibly deal with/prepare for that?
Axolotl-like regeneration will make medical care a whole lot easier; they can regenerate their eyes, spinal cord, and even parts of their brain. Combined with the immunity the Labyrinth dwellers will have for poison and disease, hospitals should become largely irrelevant, since only a few things would need to be treated (largely malnutrition and dehydration, which would be rare inside the well-planned Labyrinth, and perhaps heatstroke and hypothermia). This would force medical professionals to seek other employment, so what impact would that have on them?
If more information is needed, please let me know. As always, I appreciate your input and feedback. Thanks in advance!