I'll keep this review as spoiler free as possible.
// Story
The story is complex, hard to get, and hidden, hence the reference to dark souls in the name. If you start to like this game, you will fall into the rabbit hole (wink).
So much things are well thought out as all the gameplay mechanics have an in-game explanation. NPC's are interesting and some will give a real challenge to discover their story arc or even to be discovered.
// Replayability
Replayability is the way to go, trying different things, finding out about what really is happening and what to do. That also means the first playthrough can be frustrating, as NPC tend to disappear or die without you knowing why, and that was the case for me.
The game really begins when you finished it once, because you will stop discovering the layout of the map, but focus on what yours and the NPC's stories are.
// Difficulty - Gameplay ( and darksouls-ery)
Concerning the difficulty, the game will test you many times. Few would be given to you and if you do not like that, there is a way to grind rapidly to avoid the difficulty.
It is not the way to go though, and the last DLC3 will catch up on you anyway.
Gameplay wise, there is a lot of fun to establish strategies to beat your opponents instead of just grinding and mash one button in the fights.
Contrary to Dark Souls, as this game is made on RPG maker, all buffs/debuffs are displayed to hint you on what to do. Also, the bosses weakness or types are easy to guess.
// Music
The OST (BGM in RPG Maker language) is really cool with some bangers, and all fit really well their environments (except you Infinite Food).
// Art
Maps are well designed with intrisic details, H-scenes have a dedicated style of art, that is the same in BlackSouls. There is so much variety, many zones/biomes and in each of the screens, consistency and details.
// Content
The map is HUGE, many environment, many enemies, many bosses, many objects, many builds possible, ... the content is there, and boy does it gives ! And again, almost everything has a meaning.
When I thought I knew everything and spent some time in the comments, the theories of the players struck me. There were many theories based on details I missed because it was for me just a gameplay/environment element (and also surely because I am french and I can skip details of the english languages), and the consistency tends to make them right.
// Conclusion (finally ?!)
I thought to spend a day or two on that game, and I still today do not stop discovering new things ; even more after spending some time in the japanese wiki. It is not my type to use a translator to decipher a site to uncover what it contains, and the reward was there.
The creator of this game made such an amazing work, it has to be recognized. So like in the golden age of piracy, try it, play it, 100% it, but think that if you want more content like that, and you have some money to spare, think about funding him by paying it.
PS: Notice that I talked about the H part of this H-RPG only once in the review. It is always there, but there is so much beyond that that the game would still have been amazing without (at the loss of some character design, art, and world depiction).