So, the disaster begins with Makoto, a young man committed to becoming a sexual predator. He seduces girl after girl, until eventually he hits one who takes it badly. She summons a succubus to exact vengeance on him, but is taken by it herself first. Then they go after Makoto, who happily begins his conversion to an Incubus. Since he is willing, the succubi allow him to go out and bring back other girls to be converted into succubi. Eventually there are enough of them that they take over the town.
The outbreak leads to an open war, and ends with succubi being left dead all over the landscape. Some of the dead succubi are eaten as carrion by the slimes, goblinoids and ogres, who are mutated by the unclean flesh. Other succubi become part of the soil, and mutate the plants. The plants mutate the insects, who mutate the birds and bats.
These monster girls intrude into the population established in an older era by Echidna - the Lamia, Centaurs, Werewolves, Mermaids, Medusa, lizardmen, and Scylla.
Pixies, zombies, gargoyles, mimics, and golems are created by mages binding imps.