Long ago, an Eldritch deity was summoned to the city of Innsmouth within a ritual circle, where it was killed and consumed by the population. Devouring the flesh of the deity increased the mana supply of the individuals to astronomical levels, strengthening their arcane abilities. As the population grew in power, the prestige of Inssmouth increased in the world. As all inhabitants benefited from their mana supply increase, the city became known as a center for magecraft where the most powerful mages were located. This lasted for a period of 500 years, as the population passed down their arcane power to their descendants.
However, over time, problems started developing in the city within the past 50 years. The current population began suffering from defects and mutations.It turns out that the increase in mana from the god began to mutate future generations. Individuals with heavy deformities became twisted and inhuman, as their mental faculties decreased while becoming animalistic. More defects in the gene pool began showing up as a significant portion of the population gave birth o monsters. Ultimately, this ended with the entire city becoming filled with debased creatures with no semblance to humanity.
Somehow killing and consuming the pieces from the god had an effect on the genes of the population. This hereditary illness was passed down to their descendants, but didn't reveal itself. The chaos of the mutations occurred suddenly and quickly after centuries of stability. What would cause this massively delayed reaction?