Fate/Grand Master

From the introduction: Welcome back to Chaldea, traveller. If you’re familiar with the world of Fate/Grand Order then you already know something of what awaits you here: human history has been incinerated by the machinations of the King of Mages, leaving only the men and women of the Chaldea Security Organization left to oppose him. In another time and place it would’ve been a completely ordinary person who became the last master and led a team of servants to victory on mankind’s behalf, but here and now that’s no longer the case. For this isn’t the world of Fate/Grand Order but of the many depraved h-doujins based on it, from the Marked Girls series authored by Suga Hideo to the works of Asanagi and Bbsacon and many others like them. In these worlds the Last Master to whom this great responsibility falls turned out to be such a hopeless pervert that he even warped the rules of the FATE system to his benefit, but that perversion may be the very thing that humanity needs most.