Black Souls and
Black Souls II are the gold-standard in terms of hard-to-digest games. Like the From Software games they pay homage to, the story isn't spelled out for you, leaving you to scrounge together dialogue scraps, hidden messages, and context clues to figure out what the hell is going on. The closest game I would compare it to story and gameplay-wise is OFF. Not to mention the various fantastic secret endings and bosses which encourage repeat playthroughs.
The first Black Souls is porn-heavy compared to the second one, which is kind of a shame because the creator's art was pretty poor at this time. It's typical RPGMaker fare with a few quirks. First of all, you have to summon your party members in battle using up a turn, which is not only an investment but also downright dangerous as many of the enemies in the game can one-shot you. Second, because the enemies hit so hard, the battle revolves around the use of defensive mechanics and Active Time Battle (a la Final Fantasy). These mechanics get dangerously cheesy when you figure out how best to use them, which is somewhat rewarding as it gives you the feeling that you're beating a downright unfair game. And, lastly and possibly least (for me anyways), the porn aspect comes in because your character's cum is magical and allows you to impart strength onto them, so they have to suck your dick or whatever to get stronger. Overall, I would recommend trying Black Souls I if a little RPGM jank won't hurt you. It's not necessary to play to enjoy the second game, however.
Black Souls II is much less porn-heavy. It's still there, but the plot no longer revolves around it. It's also fucked up when you realize later in the game, especially in the DLCs, why your character's first thought when they see people is the omnipresent dialogue options of "kill" or "rape". There's also some great callbacks to the first game which you'll catch if you played that first, but they're not necessary for the enjoyment of the story. The gameplay in this one is much different, instead of relying on party summons you must use the equip swapping mechanic. Every weapon has its own set of unique properties and skills, and in order to win in the mid to late game battles, you have to understand what strengths and weaknesses all of your weapons have and how best to combo their skills together. It has a huge degree of playstyle variability.
Overall, if you couldn't tell, I really love these games. Highly recommend if you're looking for environmental storytelling mixed with RPGM horror.