A combination of isekai and harem story, written by Anonymoose and based in the world of monster girl encyclopedia. It revolves around the main character being "magically transported to a world of miracles and magic, full of all manner of creatures from folklore that have metamorphosed into beautiful women for reasons unknown, where a great conflict is brewing between the newly-crowned Monster Lord, and the old "order," who wish to eliminate the monsters and return the world to its pre-metamorphosis state."

MGS is best known for its extreme length- the story has been written continuously for a decade, and is now seven volumes long (over 120 chapters.) It can be read here: http://anonymoosesaga.pbworks.com/w/page/67867177/Introduction
monster girl saga