There is no great explanation for skulks in the world. They are clearly a cursed race and are, according to legend, created when a human city maintains neutrality during a time when the god and goddesses call for great action. A number of cities have been cited as being infested with Skulks, but most of them are as mythical as the creatures. Perhaps the curse extends past the creatures themselves and infects their homes as well. No one can say for sure.
What’s more, many skulks are born as general curses to humanity. How they survive to maturity is anyone’s guess since, basically, they were, at one time, invisible babies.
What’s for sure is that the creatures are fairly rare and, except for in certain cursed cities (if the legends are to be believed), do not constitute a race.