The game is clearly a work of love. The designer spent a lot of time and energy to make a solid product. That being said, the combat is tough, for the wrong reason. I found the combat bland and unengaging. The combat is functional and adequate just not interesting. I would love to see the author take another crack at making games while looking at different ways to make combat more engaging. Some examples could be a character which uses stances, buffs / debuffs onto self or party members, a stronger focus on unique abilities (In the hour I played, I ran into none). Some other ways in which the system could be improved could be removal of weapon proficiencies, I struggle to see how those add to more interesting gameplay. I would encourage the author to look at some other RPGMaker titles such as Black Souls, The Last Sovereign, and Renryuu: Ascension which (to me) pull off what the author of The Triumphant Return of Diabolos tried to pull off.
Overall, if a person hasn't played many titles made by RPGMaker, this title would be a great experience. For people who are veterans of hentai games made in RPGMaker the unengaging combat outweighs what would otherwise be a good game. I would rate this at a solid 7/10 and look forward to what the author's next game will be!