After the nuclear apocolypse, aliens helped repopulate the Earth. The planet was pushed out of orbit to fit with the temperatures of other creatures planets. Earth suffered and several climate apocolypses happened. The planet was fixed with the magic of the aliens. Thousands of years passed and we get the story of our current main characters. (Croco, Nixon Bubble, Bowler Skull, Rock Guide)
Since magic exists, witchs are in the population. It is unknown whether they are people corrupted by dark magic or just aliens who take the guise of people until revealed as otherwise. The main witch that terrorizes the core four of 5:00 is THE CLOCK WITCH, a witch who has gimmicks and tricks based on time themed things. Mostly she's just a jerk.
Humans are still left in a very small amount, most likely after adapting to the new climate. Half of them are mutants and the rest are most likely genetically altered to survive in the post-apocalypse.
A cult has formed based on the tantalizing glow left in the mutants skin after bathing in the time based explosions and expulsions caused by the Clock witch. The members of this cult are known as 5:00ists, since they make sacrifices to Bowler Skull, of whom's marking as 5:00 PM has caused a society, and a giant skeleton mech has been built from the large piles of skulls all over their island. The source is assumed to be from the other remnants of the human population, although the 5:00ists have the habit of digging up corpses, so they could just be assorted skulls, but my money's on the remnants of the population thing.
The diamond spa is a scam ployed by witches to take dead people's souls, stick 'em in a wax figure of their past self, realistically bring them back from the dead in a new muscular form, and then they have lifespans aided only by ambrosia and nectar.
