The Runesmith Chronicles: Oni and the Farmer

As thanks for helping him save his mother, a young farmer boy offers to become a servant to a large woman with blue skin and horns called an oni. The always-horny oni often puts his services to use firmly buried between her legs. Finding later he has mysterious abilities and a special affinity for the magical arts she starts him on the path to becoming a special type of mage known as a Runesmith. Loosely based on MGE so expect monster girls, but don't expect any moronic 'reverse rape'. https://storiesonline.net/s/19158/the-runesmith-chronicles