Nightmare Sphere has to be the best freeware clone of Castlevania: Symphony of the Night that's ever been made, and I'd say it even outdoes some of Konami's own later entries in the series, at least in terms of nailing the tone and gameplay feel of SotN. Odd, then, that it bolts on some completely gratuitous "erotic content", when the game would have stood up just fine on its own (and likely reached a much wider audience) without it. As with Holdover, another freeware sexually-charged platformer that I happened to play and review just before this one, the choice to incorporate the "H" content apparently just seems to stem from the designer's own fetishes coming to the forefront than from any sort of design or marketing plan.
So what's the fetish here? Monster rape. Both our heroine and various female NPCs can be knocked down, and when they are on the ground an opportunistic monster might dive in for a life-stealing hump session. It's actually possible to almost entirely avoid this if you don't care to see it, however. The heroine very rarely gets knocked over in the presence of a rapey monster, and there's an insta-midair-recover that's fairly easy to pull off as well. Other than that, I don't think there's any "H" content to be seen here -- not even any real nudity.
NO POCHI. BAD. DOWN POCHI.
The developer is Japanese, and the English translation is charmingly clumsy, but it gets the point across (and there's almost no dialogue after the first 60 seconds of the game anyway). You're a half-demon girl named Relm who wakes up in a demon-infested castle and has to find her way out. This entails exploring in SotN style, gradually beating bosses and gaining power-ups that let you Metroidvania out a little farther by doubling back to previous areas. You'll also level up, learn new moves that have Street Fighter-esque button inputs and that chain into combos, and gradually pick up some magic too. If you've played SotN you'll be immediately at home.
Mummy's got nards!
In addition to all this, there's also a fairly basic morality system. I don't think there's any effect to it other than determining what ending you get, but your morality meter will go up by killing certain types of monsters and rescuing NPCs from monster attacks and will go down for attacking/killing humans or sucking out their delicious life force when they're knocked down. NPCs seem to exist solely to either boost your meter or get raped, with no dialogue nor purpose other than wandering aimlessly in the areas they've been plopped into. While directly harming them is regarded as evil (even the rapey men that attack you first), there's apparently no Good Samaritan laws in this world as you're free to stand on a safe perch and watch a goblin beat them up and then rape them to death with no morality penalty whatsoever. Then you can simply exit and re-enter the screen and both monsters and NPCs have respawned like nothing ever happened.
Succubus face riding, this game has everything
While the game is amazingly polished and well thought-out for a one-man freeware perv project, it's not without significant flaws. Monsters and NPCs can't move about when they're not visible, so they'll just freeze in their last position until you scroll them back on the screen again. Gameplay is quite fluid on the whole, but there's a few small hinks here and there, like sometimes when you enter a screen you won't be able to move forward initially for a little while like you're pressing against an invisible wall. The big one is that certain areas, such as the Forest area near the beginning, are prone to lock the game up if you jump or do the wrong action in certain spots, forcing you to close the game out and lose all your progress. That latter bit eventually drove me away from the game, but had it not been so prone to freezing I probably would have actually played it to completion.